A subseasonal weather forecast attempts to predict weather conditions three to four weeks in ______ its predictions are
therefore more short-term than those of the seasonal forecast, which attempts to predict the weather more than a month in
advance.
advance,
advance
advance;
advance and
Choice C is the best answer. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are both
independent clauses, so using a semicolon to separate them is grammatically correct. 
Choice A is incorrect. This choice creates a run-on sentence error. The clause “A subseasonal…advance” and the clause “its
predictions…forecast” are both independent clauses, so a comma is not enough to separate them. Choice B is incorrect. This
choice creates a run-on sentence error. The clause “A subseasonal…advance” and the clause “its predictions…forecast” are
both independent clauses, so they need to be separated with speciﬁc punctuation (a period, a semi-colon, a colon, a dash, or
a comma + a coordinating conjunction). Choice D is incorrect. This choice creates a run-on sentence error. The clause “A
subseasonal…advance” and the clause “its predictions…forecast” are both independent clauses, so the word “and” by itself is
not enough to separate them. There would need to be a comma before “and” for this choice to work.