Integer m has 4 different prime factors and n has 3 different prime factors. If m and n has the greatest common factor of 15, how many different prime factors does mn have?  Choose the correct choice from the following answers.
A) 4
B) 5
C) 6
D) 7
E) 8
As m and n has GCF of 15, 3*5 should be common factor for both m and n.   m=3*5*p1*p2   n=3*5*p3   mn=3^2*5^2*p1*p2*p3   mn has prime numbers of 3,5,p1,p2p3   5
<result>B</result>
Integer m has 4 different prime factors and n has 3 different prime factors. If m and n has the greatest common factor of 15, how many different prime factors does mn have?