Hi guys, im currently studying for my ppl met exam, failed it last week by 1 mark

. So ive got my hands on the ppl confuser questions and one i think its got the wrong answer but was hoping someone could put my mind at rest.
Given a surface temp of +21 C and a Dewpoint of +7 C, at aprox what height will the base of the Cumulus cloud be found?
A - 4000ft
B - 7000ft
C - 8500ft
D - 5500ft
The answer in the book says D but i can't work that out to be right. Ive used the DALR as -3 C /1000ft which makes the base between 4-5000ft. Any help would be appreciated thanks.