If a is a positive integer, then 2^a + 2^(a + 1)+ 2^(a + 2)+ 2^(a + 3) =
Choose the correct choice from the following choices:
A) 3^(a + 1)
B) 2^(a + 1)
C) 2a
D) 2a^(a + 1)
E) 15(2^a)
If a is a positive integer, then 2^a + 2^(a + 1)+ 2^(a +2)+ 2^(a + 3) =   2^a(1+2+4+8)   2^a(15)   E. 15(2^a)
<result>E</result>
If a is a positive integer, then 2^a + 2^(a + 1)+ 2^(a + 2)+ 2^(a + 3) =