pronouns - "more robust than five years ago" vs. "more robust than it was five years ago"

This is from a question from GMAT.

The official correct answer is C. but I am wondering what is the difference between A and C. Both seem correct and clear to me.

10. A large rise in the number of housing starts in the coming year should boost new construction dollars by several billion dollars, ___________________.

(A) making the construction industry’s economic health much more robust than five years ago

(C) making the construction industry’s economic health much more robust than it was five years ago