In the exponential growth or decay function, explain the circumstances that cause $k$ to be positive or negative.

Choose the correct explanation below.

B$.k$ is positive when an increase in time leads to an increase of a beneficial quantity, such as money $k$ is negative when an increase in time leads to an increase of an unfavorable quantity, such as bacteria.

C$.k$ is positive when an increase in time leads to an increase in the quantity present. $k$ is negative when an increase in time leads to a decrease in the quantity present.

D$.k$ is positive when an increase in time leads to a decrease in the quantity present. $k$ is negative when an increase in time leads to an increase in the quantity present.