Which of the following best describes the shape of a plot of the number of undecayed nuclei N versus time t for a radioactive sample?
a) A straight line decreasing uniformly from left to right
b) A step-like graph that drops by one-half at each equal time interval
c) A curve that falls rapidly at first and then more slowly, approaching the time axis without touching it
d) A curve that rises rapidly at first and then levels off to a constant value