A5. In a group of 20 boys, each weighs on average $62 \mathrm{~kg}$. If a teacher joins the boys, the average weight of each is $63 \mathrm{~kg}$. How much does the teacher weigh?
(A) $62 \mathrm{~kg}$
(B) $73 \mathrm{~kg}$
(C) $83 \mathrm{~kg}$
(D) $93 \mathrm{~kg}$
(E) cannot be calculated