Manhattan GMAT Sentence Correction - Verb Tense question

[mba_aspirant911]

On page 108, 4th edition of MGMAT's SC guide, the following sentences are listed:

1) The child DREW a square in the sand, but the ocean ERASED it
2) The child DREW a square in the sand, but the ocean HAS ERASED it.

Sentence 1) describes the act of erasing in the Simple Past tense, and Sentence 2) describes it with a Present Perfect tense.

It is also stated that in the first sentence (Simple Past), the state of the sand is not known but that in the second sentence (Present Perfect), it is known.

This is a little confusing to me. I know that Present Perfect can be used to describe a continued action (e.g. He has been sick since Tuesday) or a completed action whose effects still exist now (e.g. The artist has drawn a picture).

Why doesn't 1) clearly describe the actions of the ocean erasing the square in the sand, through the words, "erased it"? From this, can't we infer that the square was indeed drawn but the ocean came along and erased it, and therefore, it no longer exists?

[grockit_andrea]

We can deduce from sentence 1 that the ocean did erase the square, but we can't be sure if anything happened after that. For instance, it would be entirely possible for the next sentence following sentence 1 to say, "Then the child returned and drew a new square in the same sand." Since all we know is that the original square was erased at one point, we can't draw any inferences about the current state of the sand. However, sentence 2 says that the ocean has erased the square, and from the present perfect tense, we can infer that the square remains erased, since the present perfect indicates a state that began in the past and continues into the present.

[mba_aspirant911]

Andrea,

Thanks very much for explaining the subtle difference through that example.

Just as an aside, would it then be awkward to follow up 2) with a sentence that describes further action? Such as:

2) Extension: "The child DREW a square in the sand, but the ocean HAS ERASED it. The child is now drawing the square again."

Instead it would be clearer to use the Simple Past to represent what happened before the current action:

1) Extension: "The child DREW a square in the sand, but the ocean erased it. The child is now drawing the square again."

[grockit_andrea]

I think that in your example, the simple past tense would be a better choice. The whole point of the present perfect tense is that it is extending into the present; if the square was erased but is now being redrawn, its erased status is effectively ended, so the present perfect is no longer applicable. That being said, it's such a minor distinction that I can't imagine it being the only difference between two answer choices on the GMAT.