When a drug is injected into a person's bloodstream, the amount of the drug present in the body increases rapidly at first. If the person receives daily injections, the body metabolizes the drug so that the amount of the drug present in the body continues to increase, but at a decreasing rate. Eventually the quantity levels off at a saturation level. Let $Q(t)$ denote the quantity of the drug in the body $t$ days after receiving the first injection.

(a) Based on this description, the function $Q(t)$ 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 $Q(t)$ is
A. Concave Up
B. Concave Down
C. Neither, it is linear
D. Both concave up and concave down