If the new airboat does what it is to be doing—travel at high speeds

Answer is B for stating the purpose of the new boat in infinitive terms and for referring the pronoun 'it' wherever it appears to the subject of the the subordinate conditional clause.
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Correct answer: B

A: Incorrect. "Is to be doing" does not mean "supposed to be doing" as the meaning of the sentence (signaled by "if") suggests. Rather, "is to be doing" negates the conditional by simply stating "if the new airboat does what it will do"--a scenario that is not at all a conditional.
B: Correct
C: Incorrect. "It does as" suggests that the sentence refers to an entity other than the new airboat. The sentence, however, clearly refers to the "new airboat" with the "it" later on in the sentence.
D: Incorrect. "Doing what it is" creates a similar error as seen in (C) but with a nonsensical meaning.
E: Incorrect. "Doing" creates a similar error as seen in (C), suggesting that the sentence is referring to an entity other than the new airboat.
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Do you mean that "to be doing" has the same meaning as "will do"? This verb phrase "to be doing" is new to me. Please elaborate on it to me.

If the new airboat does what it is to be doing—travel at high speeds undeterred by sandbars, crocodile-infested mudflats, or marshy hippo haunts—it could revolutionize transport on the 2,900-mile-long Congo River.

(A) If the new airboat does what it is to be doing
(B) If the new airboat does what it is supposed to do
(C) If it does as the new airboat is supposed to do
(D) Doing what it is the new airboat is supposed to do
(E) Doing what the new airboat is to be doing

Answer is B

Explanation :

If the new airboat does what it is to be doing—travel at high speeds undeterred by sandbars, crocodile-infested mudflats, or marshy hippo haunts—it could revolutionize transport on the 2,900-mile-long Congo River.

(A) If the new airboat does what it is to be doing --- to be doing is not correct
(B) If the new airboat does what it is supposed to do - correct- proper conveying meaning
(C) If it does as the new airboat is supposed to do It is not good way to use . and What is subject to it is representing
(D) Doing what it is the new airboat is supposed to do --It is not good way to use . and What is subject to it is representing
(E) Doing what the new airboat is to be doing --It is not good way to use . and What is subject to it is representing

Without getting into all the details, let’s first eliminate the options based on superficial meaning.

(C) If it does as the new airboat is supposed to do
Here ‘it’ seems to refer to another noun and not the ‘new airboat’. Eliminate.

(D) Doing what it is the new airboat is supposed to do
‘Doing what it is’ does not make any sense here. This is awkward. Eliminate.

(E) Doing what the new airboat is to be doing
‘doing what… doing’ is not elegant. Also suggests that a new subject will be introduced in the main clause, which is not the case here. Eliminate.

Now we can come back to A and B which are really close and possibly where we can get stuck.

(A) If the new airboat does what it is to be doing
The structure of the sentence here is [if clause]+[main clause] and the intent of the if clause is to express a condition: if the boat does what it is expected to do. This option does not completely satisfy the condition that ‘if’ introduces. Eliminate.

We see this clearly expressed in Option B.


B is the best option.

1. "to be doing" is a more complex form of "is doing". And "to be doing" is simply unnecessary.

2. we have to make "new airboat" the subject. Otherwise, the "it" in D would have no precedent.

3. the "could" in the non-underlined part implies us to use conditional. Notice: we usually have 2nd conditional sentence which is: past tense in "if" clause, and could/would/might in "then" clause. For example, If I knew her phone number, I would/could tell you.
However, in this problem, we have present tense in "if" clause, and that is because we are making an assumption/hypothesis on the current fact/truth.
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