For a,f,d are the positive integers, and d|a means that “a is divisible by d”, if d|af, which of the following must be true?  Choose one
A) d|a B) d|f C) d|2af D) d|(a+f) E) d|(a-f)
d/a means a is divisible by d   d/af means af divisible by d.   all are integers so ,   if af is divisible by d,   1 a can be divisible by d   or   2 f can be divisble by d.   so the question stem asks must true.   so option a and b are could but not must.   option c is 2af divisible by d. if ab is divisible by d then 2af is divisible by d.   option d and e we can't predict.   so option d|2af is correct.
<result>C</result>
For a,f,d are the positive integers, and d|a means that “a is divisible by d”, if d|af, which of the following must be true?