Which option is grammatically correct?
(a) She had practiced the set of drills frequently; so she was able to execute the routine flawlessly.
(b) She had practiced the set of drills frequently, so she was able to execute the routine flawlessly.
(c) She had practiced the set of drills frequently so she was able to execute the routine flawlessly.
(d) She had practiced the set of drills frequently so, she was able to execute the routine flawlessly.
Correct answer: B
Explanation: In this example, “so” indicates that the second part of the sentence is a consequence or result of the first part of the sentence. It is therefore preceded by a comma, which logically separates the consequence from the cause.
Tip for ESL writers: “So” can also be used to signify a motivation for an action, in which case the punctuation is different: no comma is required. Sentence C can be edited slightly to use this meaning of “so”:
She practiced the set of drills frequently so that she would be able to execute the routine flawlessly.
In this case, the consequence (she would be able to execute the routine flawlessly) is the girl’s aim, but we have no way of knowing whether it actually happened or not (sentence B we know it did happen).