several calculus problems from homework assignment that i couldn't solve

1. A particle moves on a line according to the law s=f(t) so that its velocity v=ks, where k is a nonzero constant. Its acceleration is

(A) vk^2

(B) sk^2

(C) k

(D) 0

(E) none of these

2. As the tides change, the water-level in a bay varies sinusoidally. At high tide today at 8 A.M., the water-level was 15 feet; at low tide, 6 hours later at 2 P.M., it was 3 feet. How fast , in ft per hr, was the water-level dropping at noon today?

(A) 3

(B) (π√ 3)/2

(C) 3√ 3

(D) π√ 3

(E) 6√ 3

if you could answer any of these question, help me please