The Lewis and Clark expedition was to follow the Mississippi to its so

The Lewis and Clark expedition was to follow the Mississippi to its source, look for easy water routes across the continent, continue to the Pacific, and return.

A. look for easy water routes across the continent, continue to the Pacific, and return.
B. to look for easy water routes across the continent, continue to the Pacific, and then return.
C. to look for easy water routes across the continent, to continue to the Pacific, then returning.
D. look for easy water routes across the continent, to continue to the Pacific, and to return.
E. look for easy water routes across the continent, continue to the Pacific, then returning.

You can see the path to the correct answer in the following way.

Look at the beginning of the sentence.

The Lewis and Clark expedition was to

OK, "was to" what?

follow ..., look ..., continue ..., and return.

A works perfectly.

Let's consider B.

The Lewis and Clark expedition was to follow the Mississippi to its source,

B. to look for easy water routes across the continent, continue to the Pacific, and then return.

We have "to follow" and "to look". So, we need a "to" before each of the other items in the list, but "continue" and "return" do not have "to" before them. So, this list is not parallel.

Let's consider C.

The Lewis and Clark expedition was to follow the Mississippi to its source,

C. to look for easy water routes across the continent, to continue to the Pacific, then returning.

In this version we have a list that has no conjunction. "then" before "returning" does not serve as a conjunction in the way that "and" would in that spot. Also, all of the items in the list except for "returning" are in infinitive form. So, the list is not parallel.

Let's consider D.

The Lewis and Clark expedition was to follow the Mississippi to its source,

D. look for easy water routes across the continent, to continue to the Pacific, and to return.

D is a trap. Because "look" is at the beginning of the underlined portion, It's easy to miss the fact that "to follow", "to continue", and "to return" all include "to", while "look" is alone and is not preceded by "to". Since the elements of the list are all in the same form except for "look", the list is not parallel.

Let's consider E.

The Lewis and Clark expedition was to follow the Mississippi to its source,

E. look for easy water routes across the continent, continue to the Pacific, then returning.

This version almost works. We have "The Lewis and Clark expedition was to

follow ..., look ..., continue ...,"

So far, the sentence works. However, it fails at the end. Rather than a conjunction and another word that works with "was to" we have "then returning". So, we don't have a conjunction to hold the list together, and "returning" does not work with "was to" the way the other three elements of the list do.

The correct answer is A.