C. Whales. Because normally it is the first word and even though mammals would be considered a subject, it isn't the main subject.
Hope this helps!! :)
The correct version of these two sentences when combined is:
When we were in Europe on vacation, we usually flew on airplanes: however, sometimes we took the train.
We apply the semi-colon to indicate that they have different ideas, and that they are clauses being joined into one sentence.
The Greek god is Dionysus; he was the youngest of the Greek gods, supposedly and was the last to be admitted into Mt. Olympus.
