What are the next letters in the following pattern?

at, cs, fr, jq,

A. pp

B. pn

C. on

D. op

answer is OP

count the difference in letters between the first letters

a - c-f-j c is 2 away from a, f is 3 away from c, etc so the next letter in that pattern would be O

the second letter is going backwards tsrq, so P comes before q