Question

Which sentence contains a dangling modifier?

Answer 1

The correct answer is option b. Upon hearing the fire engines, the apartment building was evacuated.

A dangling modifier is a type of modifier that describes a word or phrase that is not clearly stated in the sentence, making the meaning of a sentence unclear. The sentence 'Upon hearing the fire engines, the apartment building was evacuated.' does not specify who heard the fire engines. In fact, there is nothing in the sentence to which the modifying phrase 'Upon hearing the fire engines' can refer; consequently, it is a dangling modifier.

Answer 2

The correct answer is B. Upon hearing the fire engines, the apartment building was evacuated.

A dangling modifier is a type of a modifier which is incorrectly placed within a sentence, or doesn't refer to anything. So, as you can see in the example above, the dangling modifier upon hearing the fire engines is incorrectly used - it would seem that the apartment heard the fire engines, whereas that is obviously not the case. 
A correct revision would be: Upon hearing the fire engines, the firefighters evacuated the apartment building.