Take another look at the original sentence:

Gustavo can tolerate any lecture, no matter how boring the instructor is, in his mind, Gustavo imagines that he is on board the starship Enterprise, blasting Romulans from his post on the bridge.

You wanted to fix it this way:

Gustavo can tolerate any lecture, no matter how boring the instructor is; In his mind, Gustavo imagines that he is on board the starship Enterprise, blasting Romulans from his post on the bridge.

A semicolon is a great choice for fixing a comma splice. When you use this option, however, you must use it correctly. Unless the word following the semicolon is a proper noun, one that always begins with a capital letter, do not capitalize it.

You might want to consult the rules for fixing comma splices and fused sentences.

Go back to the sentence to try again.

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