7. Find the sum of the digits of all numbers in the sequence $1,2,3,4, \ldots, 1000$.
(A) 4501
(B) 12195
(C) 13501
(D) 499500
(E) None of the above
7. Answer: (C)
Among all numbers with 3 or less digits, each $i, i=0,1,2, \cdots, 9$, appears exactly 300 times. Thus the sum of the digits of all the numbers in the sequence $1,2,3,4, \ldots, 999$ is
$$
300(1+2+\cdots+9)=13500,
$$
and so the answer is 13501 .