Which sentence demonstrates an error in parallel structure?

Question

A.
After driving around the city, I learned that the library is on Heartz Street, the post office is on Bethel Road, and, conveniently, the city pool/recreation center is on Parkway Boulevard.
B.
Smoke oozed from the volcano's peak, the ground vibrated, and, with a thundering roar, a cloud of fire shot into the air.
C.
The Web site needs to be changed to describe what time the game begins, how to get to the stadium, and, to prevent unnecessary citations, where to park legally.
D.
At my last appointment, my dentist talked to me about removing my wisdom teeth, filling my cavity, and, less importantly, how to whiten my smile.

Answer 1

The sentence that demonstrates an error in parallel structure is:

D. At my last appointment, my dentist talked to me about removing my wisdom teeth, filling my cavity, and, less importantly, how to whiten my smile.

In this sentence, "removing my wisdom teeth" and "filling my cavity" are gerund phrases, while "how to whiten my smile" is a clause, which disrupts the parallel structure. To maintain parallelism, all items in the list should be in the same form. For example, it could be revised to: "removing my wisdom teeth, filling my cavity, and whitening my smile."