When a rumor begins, the number of people who have heard the rumor increases slowly at first. As the rumor spreads, the rate of increase gets greater (as more people continue to tell their friends the rumor), and then slows down again (when almost everyone has heard the rumor). Let $N(t)$ denote the number of people who have heard the rumor $t$ hours after the rumor has begun.

(a) Based on this description, the function $N(t)$ is
A. increasing
B. decreasing
C. neither increasing nor decreasing
D. both increasing and decreasing

(b) Based on this description, the graph of the function $N(t)$ will be
A. Concave Up
B. Concave Down
C. Neither, it is linear
D. Both concave up and concave down