Correct statement about random process of radioactive decay

[songoku]

Homework Statement

A physics student reads in a textbook that radioactive decay is a random process. This means that, for a sample of a given radioactive isotope, we cannot tell
A) what the radioactive isotope will decay into.
B) when the sample will start to decay.
C) which radioactive nucleus will decay next.
D) which type of radiation will be emitted.

Relevant Equations

none

The correct answer is (C) but I don't understand why (B) is wrong. Isn't (B) also the meaning of random process? I though (B) and (C) are both correct.

Thanks

 

[anuttarasammyak]

A sample made of a lot of radioactive nuclei started to decay immediately after it was generated.

 

[songoku]

Thank you very much anuttarasammyak

 

[kuruman]

Choice (B) is a bit ambiguous. To go to extremes, Tellurium-128 has a half-life of $7.7\times 10^{24}$ years via double beta decay. If I am given a sample, I "know" theoretically that it has started to decay but, arguably, the sample "has started to decay" as soon as I observe the first decay products which may or may not be in my lifetime.

 

[haruspex]

My objection to the question is that C is only part of what it means. So yes, you can arrive at C by elimination of the offered alternatives, but it also means there is no instant at which a decay can be predicted to occur. E.g., C would permit that a decay occurs exactly once every second, we just don’t know the order in which the atoms will decay.

It is also true, of course, that the likelihood at time t of a given undecayed atom decaying in the next interval of duration T depends only on T, not on t. Saying it is random is sometimes interpreted as implying that, but strictly it does not. The appropriate description of that is "memoryless".