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

I Like option c) or d) for the first one. But why?

Quote:

Originally Posted by pickslides

I Like option c) or d) for the first one. But why?

$a=\frac{dv}{dt}=k\frac{ds}{dt}=kv=k\times ks=sk^2$

Tha answer is (b).