how to use the one & it, 求教98-08-14

waterfrog

代词it要指代一个之前提到过的东西，因为原物在句子里面没有提到过，所以用the one

groundkeeper

The lenses in an optical microscope bend the light passing through a specimen to form an image of that is much larger --- actually viewed.

(A)than it

(B)than the one

(C)one than

(D)than one which

answer is B, but how to use it and the one in what condition? and the difference between them.

thanks a lot!