Hello, I am confused about the difference between 'as 'and 'than'.
A. You are not so lazy as he.
B. Mary is prettier than she.
C. You have the same trouble as I.
Why is the B wrong? The reference book says that A and C leave out (as he is lazy, as i have trouble).And the B's correct form is (she is or her). Why can't the B leave out?