The sentence appears to be flipped. The subject comes last. If you forget the modifying clause of "an industry marked by rapid innovation and volatile demand" and focus on the remaining part of the sentence its easier to parse the sentence.
Because of the enormous research and development expenditures required to survive in the electronics industry,
an industry marked by rapid innovation and volatile demand,
such firms tend to be very large.
Take the subject part of the sentence. "such firms tend to be very large." .... "Because of the enormous research and development expenditures required to survive in
the electronics industry,".
Because is a Subordinating Conjunctions. A subordinating conjunction introduces a dependent clause and indicates the nature of the relationship among the independent clause(s) and the dependent clause(s). The most common subordinating conjunctions are "after," "although," "as," "because," "before," "how," "if," "once," "since," "than," "that," "though," "till," "until," "when," "where," "whether," and "while."
"such firms tend to be very large" is a complete independent clause. An independent clause (or main clause) is a clause that can stand by itself, also known as a simple sentence. An independent clause contains a subject and a predicate; it makes sense by itself. When "because" is added the dependent clause also has to make sense and also must have a subject. The sentence as written does not have subject. B & E are the only answer choice that introduce a subject in the dependent clause. But B has the more appropriate preposition of "of" instead of "for".
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