
Ed M. answered 02/19/16
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Help with grammar, French, SAT Writing, the TOEFL and ESL.
I would say the correct answer is "b) I had tea because there was no coffee left" because this sentence has both a main/independent clause, namely I had tea, and a dependent/subordinate clause, because there was no coffee left. The presence of the subordinator because introducing the second clause is the only thing that makes this clause dependent, i.e., it thus cannot be a sentence by itself.*
*Or at least by the general conventions of Standard Written English; even in more formal registers, one will still find many single-clause sentences that begin with Because, e.g., Why are these animals suffering? Because no one cares enough. And of course Because I said so! is a staple of the Mom-ese social/occupational dialect of English.