If the first five questions are all right and the last five questions are all wrong, then f(5)=2f( 10)= 1, a holds.

If the first eight questions are all right and the last two are all wrong, then B is established.

If the first question 1 is wrong and all other questions are correct, C holds.

F (8) < f(9) means that some of the first eight questions are wrong and the ninth question is right. Whether question 10 is correct or not, F (9) and f( 10) cannot be equal.

So choose D.