The number of plant and animal species that humans are known to have

Idiom.

"Known" + to

Humans are known to have done something. Only A gets it right.

The construction here is testing the idiom ‘known to’. The word ‘humans’ should come immediately after ‘that’ in the sentence for it to make sense.

Option A is the only option that has this construction.

Option A is the best answer.

Hope this helps!

The number of plant and animal species that humans are known to have forced to extinction in the last half millenium is over 800.

The best way to answer this question is not to remember idioms but to check the meaning of the sentence.
Clue no.1:
Last half millenium . Past tense or present perfect (based on meaning)

(A) humans are known to have forced to extinction
Keep it

The number of plant and animal species (that humans are known to have forced to extinction in the last half millenium ) is over 800.

(B) humans are known as having forced to become extinct
Known as ….humans (noun) are known as (nouns).. having forced - not a noun . Eliminate

(C) humans are known for forcing to become extinct

If u r not sure about known for.. you can eliminate based on forcing. We are talking about past tense. So, forcing is wrong. Eliminate

(D) are known that humans are forced to become extinct
Humans are forced. I understand humans are forced by something to become extinct. Meaning error

(E) are known as having been forced to extinction by humans
Same error as in E
So, I picked A based on elimination and meaning based approach

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