The following sentence contains either a single error or no error at all. If the sentence contains an error, select the one underlined part that must be changed to make the sentence correct. If the sentence contains no error, select choice E.
(A) *It weaves* across a strip of tropical land (B) *where*
the Isthmus of Panama (C) *narrows* in the shape of a
long flattened letter S, the Panama Canal (D) *links*
the Atlantic and Pacific Oceans. (E) *No error*
Actually, I'd bet this is not from an actual SAT, but rather from an "SAT practice" book published outside the US.
As others have told you, the answer is most probably A.
I think they are just testing your knowledge of rudimentary grammar, and I don't believe the actual SAT tests grammar.
"It weaves across a strip of tropical land where the I. of P. narrows in the shape of a long flattened letter S"
This is a full sentence. It has a subject (it), a verb (weaves), and a subject (albeit a complex subject modified with prepositions and qualified with adjectives--the rest of the sentence).
Full sentences are always followed with a period (or a full-stop, if you are British). It is not followed by a comma, as here.
What follows after the comma is also a full sentence by itself.
If you want to join what would otherwise be two separate present tense sentences with a comma, the former clause must be turned from a sentence (in this case written in present perfect) to a clause written in the present participle.
To turn the sentence into a clause in the present participle, you would change "It weaves" to "weaving", as others have pointed out.
Anywho....change study books. This one doesn't seem too good...SAT doesn't test grammar.