Sentence Correction: Subject Verb Agreement - A Complex Subject/Long Distance between Subject and Verb

Meteorologists predict that rainfall in central southern Africa will be much lower this year than last as the extreme anomalies of positive sea level pressure measured at Darwin between March and June is a reliable forecaster of drought. 

Incorrect.

This answer choice is grammatically incorrect. The singular verb is does not agree with the plural subject anomalies.

[[snippet]]

Well done!

This answer choice corrects the Subject Verb Agreement mistake in the original sentence by changing the singular is to the plural are, so it agrees with the plural subject anomalies.

Incorrect.

While this answer choice corrects the Subject Verb Agreement mistake in the original sentence, it is stylistically flawed. The phrase drop greatly this year below last year's is wordy and redundant.

Incorrect.

This answer choice is grammatically incorrect. The singular verb is does not agree with the plural subject anomalies.

[[snippet]]

In addition, this answer choice introduces a comparatives mistake, by comparing two things that are not grammatically parallel, i.e. not the same part of speech: this year with for last. In the original sentence the comparison between the modifier this and the modifier last is correct - they both modify year (this year, last [year]).

Incorrect.

While this answer choice corrects the Subject Verb Agreement mistake in the original sentence, it is grammatically incorrect. By eliminating the reason conjunction as, the sentence becomes a run-on sentence: there is nothing connecting the two clauses:

Meteorologists predict that rainfall in central southern Africa will be much lower this year than last,
[?]
the extreme anomalies of positive sea level pressure measured at Darwin between March and June are a reliable forecaster of drought. 

Note that a comma cannot connect two clauses.

be much lower this year than last as the extreme anomalies of positive sea level pressure measured at Darwin between March and June is
drop greatly this year below last year's as the extreme anomalies of positive sea level pressure measured at Darwin between March and June are
be much lower this year than for last as the extreme anomalies of positive sea level pressure measured at Darwin between March and June is
be much lower this year than last, the extreme anomalies of positive sea level pressure measured at Darwin between March and June are
be much lower this year than last as the extreme anomalies of positive sea level pressure measured at Darwin between March and June are

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