Which of the following statements is NOT TRUE regarding sub$-$algorithms?

a$.$

A structured algorithm is broken down into sub$-$algorithms.

b$.$

Sub$-$algorithms perform specific functions and are thus more understandable.

c$.$

Sub$-$algorithms are written once in the program, and can be executed multiple times.

d$.$

A sub$-$algorithm cannot further be divided into more sub$-$algorithms.