Multipul Inheritance in C#

Question

Hi,
I am little confused , How to solve the following senario.
Let's say I have a class
ClassA{
X();
Y();
}
And classB{
Z();
}
 
Now i want to create a classC which have Method X(),Y() and Z();
But the limitation is i cant change anything of classA and classB
Can any one help me ?
Regrads
Manas