During the second half of the last century, as airplanes replaced ship

Spoiler - I got this question wrong - the sentence seemed awkward to me, a bit long with too many modifiers and commas and I had difficultly identifying the subject and determining what was an independent clause and dependent, if any. After a closer look at the answer, some research, and the description provided by ramannanda9, I disagree with his analysis. Here's why:

Here's the correct sentence "During the second half of the last century, as airplanes replaced ships as the means of transportation, it was widely anticipated that the time spent in transit by the average traveler would decrease considerably."

IMO the subject of the sentence is "It" which is apart of an independent clause of "it was widely anticipated that the time spent in transit by the average traveler would decrease considerably." Of all the answers choices this is the most preferable way to communicate this intended meaning.

IMO the sentence is introduced by a prepositional phrase that modifies another prepositional phrase. The prep. phrase "During the second half of the last century" modifies "as airplanes replaced ships". Traditionally a modifiers will modify the noun closest to it but the phrase is a prep. phrase beginning with "as". If you want to check the logic of this assertion, just flip the modifier and see if it makes sense.

as airplanes replaced ships During the second half of the last century as the means of transportation, ....

In addition, the correct sentence uses a passive construction. Let me explain. The subject is "It", but "It" is a pronoun. What is the antecedent? IMO it's "Time". A concise uncluttered active sentence would read as follows: The time spent in transit by the average traveler was widely anticipated to would decrease considerably.