15) If for every $x, f(x)=4^{x}$. Then $f(x+1)-f(x)$ is:
(A) $f(x)$
(B) $2 f(x)$
(C) $3 f(x)$
(D) 4 (E) 1
15) The answer is $(\mathrm{C})$

We have $f(x+1)-f(x)=4^{x+1}-4^{x}=4 \cdot 4^{x}-4^{x}=3 \cdot 4^{x}=3 f(x)$.