Critical Reasoning

A gas tax of one cent per gallon would raise one billion dollars per year at current consumption rates. Since a tax of fifty cents per gallon would therefore raise fifty billion dollars per year, it seems a perfect way to deal with the federal budget deficit. This tax would have the additional advantage that the resulting drop in the demand for gasoline would be ecologically sound and would keep our country from being too dependent on foreign oil producers.

Which one of the following most clearly identifies an error in the author's reasoning?

(A) The author cites irrelevant data.
(B) The author relies on incorrect current consumption figures.
(C) The author makes incompatible assumptions.
(D) The author mistakes an effect for a cause.
(E) The author appeals to conscience rather than reason.

This is an LSAT question. I chose B.
The OA is: C

Isn't B saying the same thing as C, but is more specific and relevant to this question?

The incorrect assumption are the consumption figures. Can anyone explain why B is wrong?