Programming in C

Which of the following are true about C functions?
a. C functions are used to avoid rewriting same code again and again in a program.
b. There is no limit in calling C functions.
c. We can call a function from any other function in a program.
d. A large C program can easily be tracked when it is divided into functions.
e. The core concept of C functions is reusability.
f. A function can call itself.