How many different four-digit numbers can you make using all the digits in each set only once?

a. 1,2,3,4 b. 0,1,2,3 c. 0,0,1,2

a. 4*3*2*1

b. zero is not normally a leading digit, so it cant be used: 3*3*2*1
c. 2*2*1 Think this one out.