Modern-day snakes are believed to evolve from four-legged lizards, but

Official Explanation

(1) Take a First Glance (5 seconds)

The underline begins with variations in the verb evolve and ends with either were or are, indicating that the sentence is testing Verbs. Note the timeline of the actions in the sentence to ensure the verb tenses make logical sense.

(2) Read for Meaning

The sentence describes a current belief about the evolution of snakes, as well as an ongoing debate scientists are having on that topic. In the original answer, the phrase are believed to evolve implies that snakes currently change from four-legged lizards into serpents. It is more accurate to use the present perfect have evolved to indicate that the evolution started in the past (beginning with the four-legged lizards of the past) and has continued into the present (resulting in the snakes of today). Additionally, scientists are presently divided over the land-versus-sea evolution debate (…the ongoing debate), so the past tense were divided is not correct. Eliminate answer choice (A) and any other answers that make either of those mistakes.

(3) Find a Starting Point

Start with any difference that seems easiest to you, then move to the next easiest issue, and so on. Stop when you have one answer or you aren’t sure how to address the remaining differences. All errors for each choice are detailed in the next section.

(4) Eliminate (and Repeat)

(A) This choice is incorrect as it repeats the original sentence.

(B) CORRECT. The present perfect have evolved correctly indicates that the evolution started in the past and has continued into the present. Also, the present tense are divided makes it clear that the scientists continue to be divided, as there is an ongoing debate.

(C) The present tense evolve has the same meaning error as the original sentence.

(D) Scientists are presently divided over the land-versus-sea evolution debate (…the ongoing debate), so the past tense were divided is not correct.

(E) The present progressive be evolving is not correct, as it suggests that the change from lizard to snake is still occurring. Snakes do not presently have legs, so that phase of their evolution is complete. It is more accurate to use the present perfect have evolved. Moreover, the past tense were divided is not correct, as there is an ongoing debate.

Answer: B