one princeton SC qquestion -- NN plz help

Why E is wrong, can -ing has subject "a president, a senator, and a governor"?

For B, is it clear who refers to all three passengers or only refers to "a governor"?

thx

Sorry, that is what I have from princeton. Anyway, here it is:

The first political passengers on modern railroad cars were a president, a senator, and a governor that had been traveling to Washington, D.C. together on the Liberty Express in 1907.

A. The first political passengers on modern railroad cars were a president, a senator, and a governor that had been traveling

B. The first political passengers on modern railroad cars were a president, a senator, and a governor, who traveled

C. A president, a senator, and a governor were the first political passengers on modern railroad cars who traveled

D. A president, a senator, and a governor who were the first political passengers on modern railroad cars to travel

E. A president, a senator, and a governor were the first political passengers on modern railroad cars traveling

Why E is wrong, can -ing has subject "a president, a senator, and a governor"?

For B, is it clear who refers to all three passengers or only refers to "a governor"?

The key from princeton is B.

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