Which recursive definition could be used to generate the sequence {1, 2, 2, 4, 8, }?
a) a1 = 1, and an = 2(an-1) + 2
b) a1 = 1, and an = 2(an-1) + 2n
c) a1 = 1, a2 = 2, and an = (an-2)(an-1)
d) a1 = 1, a2 = 2, and an = 2(an-2)(an-1)