下列词语中加点的字注音全部正确的一项是（ ）

Tom has failed again in the math exam. Iam really let down.—_____.

Are you getting a new car this year? — You must be joking! I can’t afford to pay my bills, _____ buy a new car. [ ]

There is some doubt _________ Mr Zhang will sign the contract with us.

The book was written in1946, the education system has witnessed great changes[ ]