How many strings of five decimal digits

a. begin with an odd digit?

my answer)

b. do not contain the same digit twice?

my answer)

c. have exactly three digits that are 9s?

my answer)

that's 5.2=10 places for remaining 9 digits

and 9 choices for each place = 10x9 = 90, but,

can't it be like this....

exactly 3 same digits means ?

d. contain exactly three distinct digits?

my answer) 10.9.8.7.10