9. Mary sells machines, on the first day she sells 1 unit, on the second day she sells 3 units, on the third day she sells 5 units, and so on, that is, she sells 2 more units each day than the previous day. Then after 20 days, she has sold ( ) units in total.
(A) 39
(B) 40
(C) 210
(D) 400
(E) 401
9. D.

From the condition, we know that the sequence of the number of machines sold $\left\{a_{n}\right\}$ is an arithmetic sequence with the first term being 1 and the common difference being 2.
Thus, $a_{20}=1+(20-1) \times 2=39$.
Therefore, $S_{20}=\frac{20\left(a_{1}+a_{20}\right)}{2}=400$.