Dear bek_gmat,

I'm happy to help with this.

This is a very tricky topic --- it's understandable that you're confused.

Rule #1: we use "like" for isolated nouns, and "as" for full noun + verb clauses---
(The technical way to say that is: "like" is a preposition, and "as" is a subordinating conjunction.)

1) Like my dog, I get excited when the mailman comes.

2) I enjoy reading poetry as my dog enjoys chewing a bone.

Rule #2: the verb "act" is a special case --- the idiom "to act like" means "to behave as, to comport one's self as" ---- this can properly be applied only to a person (or conceivably to a very intelligent animal, if we were able to attribute to it imitative behavior). We categorically cannot apply the idiom "to act like" to an inanimate object, so we need "act as". Notice, the general rule, Rule #1, is almost always the case: this is just a bizarre idiom concerning this particular verb.

Thus, "the breakwater of rocks" must "act as a buffer."

This is one of those cases in which there's a valid general rule, but the specific idiom trumps the general rule.

Here's a related blog you may find helpful.
https://magoosh.com/gmat/2012/gmat-sente ... omparison/

Does that make sense? Let me know if you have any further questions.

Mike