ACT English Question

I was going through one of the released practice tests from ACT yesterday, and I missed this question. I was hoping someone would have some insight as to why the correct answer is correct

Here is the sentence in question verbatim:

I was sure that girls growing up today would have more up-to-date role models and my generation’s favorite sleuth would of been retired to the library’s dusty back rooms.

What would best fix the asterisk portion? The two answer choices I was stuck with are:
B. would have been
D. DELETE the underlined portion.

The correct answer is B, but I chose D, and here’s why. If you left the verb “would have been” in the sentence, you would create a second independent clause. As a result, to be grammatically correct, a comma would be required before the “and” in the sentence due to the rule that two independent clauses must be separated by a comma and a coordinating conjunction (in this case “and”). Because I wouldn’t think ACT would put a grammatical error in the ENGLISH section, I said remove the verb so that there wouldn’t be two independent clauses, thus not breaking any fundamental rules. I realize that keeping the “would have been” sounds better, but I just thought that ACT would remember the comma and coordinating conjunction for this.

Any insight and/or tips for future tests?