D. parallel structure just means that all of your verbs in the comma series line up in regards to tenses, here. in choice D, it's "BE friendly," "INTRODUCE yourself," and "SMILE." these are all present tense verbs.
the rest of the choices don't match--"exercise" and "stretching" are two different verb forms in choice A, "to go to the library" is the odd one out in choice B, ane "to build" doesn't match with the "-ing" verbs in choice C.
Scientists had begun to debate privately whether or not Pluto is a plant, but this was the first time the concept was presented in a museum for all to see
B,makes more sense especially if you asked a question in the first paragraph!
