A7. Which number is next in the sequence: $144,121,100,81,64 \ldots$
(A) 47
(B) 19
(C) 36
(D) 49
(E) 50
A7. We observe that in the sequence, perfect squares appear: $144=12^{2}, 121=11^{2}, 100=10^{2}, 81=9^{2}$ and $64=8^{2}$, so the next number is $49=7^{2}$.