Look up Customer exit for variable

Hi,

I have 5 variables a1,a2,a3,b,c. a* and b are user entry vriables and c is a customer exit variable.

I need to write a customer exit such that

i) when user enters 1 for b C should take a1 as its value

ii)when user enters 2 for b C should take a2 as its value

iii)when user enters 3 for b C should take a3 as its value.

Can somebody help me with the code?

Thanks