33 - 56 The first five positive integers in binary are $1_{2}, 10_{2}, 11_{2}, 100_{2}$, 101, what is the number $10011_{2}$ in decimal?
(A) 19 .
(B) 40 .
(C) 10011 .
(D) 11 .
(E) 7 .
(8th American High School Mathematics Examination, 1957)
［Solution］ By the property of binary numbers, we have
$$
10011_{2}=1 \cdot 2^{4}+0 \cdot 2^{3}+0 \cdot 2^{2}+1 \cdot 2^{1}+1 \cdot 2^{0}=19 \text {. }
$$

Therefore, the correct choice is $(A)$.