Because you set c to zero right before your if/else statements.
c = 0;
Then you say "If c equals 1, then a = 10"
if c ==1
a = 10
This is not true, because c = 0.
And then you say "If thats not true, but c equals 2, then a=100"
elseif c ==2
a = 100
This is also not true, because c does not equal 2... c = 0. You don't have any more 'else' statements so nothing happens and 'a' is never set.
