The simplest example of the reason for C is that you can never accurately express pi in a C program.
As for B, it is simply to drill a small horn in the text. It can be said that it is wrong because real variables are not allowed to store integers. A= 10 actually converts 10 into a real number 10 and stores it in A. This is two steps, one is conversion, and the other is storage. Conceptually speaking,
(The above is purely personal.)