_______ in North American waterways a little over a decade ago, zebra mussels have already earned a nasty reputation for their expensive habit of clogging water pipes in the Grat Lakes area.

Although first sighted

The main subject and verb are situated after the middle comma. The middle comma suggests that an opening connector or verb phrase is missing. 1. It is not (A) because it has no connector. 2. It is not (C) because it repeats the main subject. 3. It is not (D) because the created meaning would be incongruent with the main sentence. 4. It is (B).