The correct answer is <span>A. Escaping the pull of gravity and traveling at high speeds both affect the rate at which time passes. If a sentence has parallel structure, it means that some of the words in it have the same form. In sentence A, as you can see, the words <em>escaping </em>and <em>traveling </em>both have the form of gerund (verb + -ing), therefore they have the same structure, thus making this sentence a parallel one. </span>