Which of the following is false?
a. A number that is divisible by 3 and 20 is divisible by 60.
b. A number that is divisible by 4 and 15 is divisible by 60.
c. A number that is divisible by 5 and 12 is divisible by 60.
d. A number that is divisible by 6 and 10 is divisible by 60.