Which Statement, A-D, Regarding the Activity of a Radioactive Sample

Which statement, A-D, regarding the activity of a radioactive sample, is not correct? If all are correct, respond E.

A) The activity of a sample is the number of decay events per second.
B) The activity depends on a rate constant for the decay and the amount of the radioisotope in the sample.
C) Radioactive decay follows first-order reaction kinetics.
D) The activity will decrease to half its initial value in a time equal to ln(2) /k, where k is the rate constant.
E) A-D are all correct.