Error while initializing a field (of base class) through constructors of derived classes.

Question

Today, I'm revising multiple inheritance for my CS board exams and I've a code which has the following structure:

A (base)
|---B (derived)
|---C (derived)

Here is my code: https://code.sololearn.com/cmOdPHTM3y9R/#cpp
In this, B uses member initializer list and C uses simple initialization. But, the initialization of B gives out an error:

"error: class 'B' does not have any field named 'num'"
I don't get why it says so, because both B and C publicly inherits public members of A. Please explain.