For this question and the next, four sentences are given, Select the option that has been written according to the requirement of standard English.

A. We'll need to do a series of tests before we do anything else.
B. We'll need to do series of test before we do anything else.
C. We'll need to do the series of test before we do anything else
D. We'll need to do series of tests before we do anything else  

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Correct Answer: D