For the positive integers \(a, b,\) and \(k,\) \(a^k\parallel b\) means that \(a^k\) is a divisor of \(b,\) but

[Gmat_mission]

For the positive integers \(a, b,\) and \(k,\) \(a^k\parallel b\) means that \(a^k\) is a divisor of \(b,\) but \(a^{k + 1}\) is not a divisor of \(b.\) If \(k\) is a positive integer and \(2^k \parallel 72,\) then \(k\) is equal to

(A) 2
(B) 3
(C) 4
(D) 8
(E) 18

Answer: B

Source: Official Guide