AMC 8 1990 Problem 1

What is the smallest sum of two $3$-digit numbers that can be obtained by placing each of the six digits $4,5,6,7,8,9$ in one of the six boxes in this addition problem?

$\text{(A)}\ 947 \text{(B)}\ 1037 \text{(C)}\ 1047 \text{(D)}\ 1056 \text{(E)}\ 1245$