License plates in a particular state are to consist of 4 digits followed by 2 uppercase letters.

​a) How many different license plates can there be in this state if repetition of letters and numbers is​ permitted?

​b) How many different license plates can there be in this state if repetition of letters and numbers is not​ permitted?

​c) How many different license plates can there be in this state if the must be ​, and repetition of letters and numbers is not​ permitted?

​d) How many different license plates can there be in this state if the first digit cannot be ​, and repetition of letters and numbers is not​ permitted?