40 A city's telephone bureau found that one user (each user owns one phone, each two users can have at most one call, phones are not lent out, and the city has no public phones) had at least 1993 calls with other users. It was also found that if two users made the same number of calls, these two users definitely did not have a common call recipient. Based on these conditions, it can be concluded that:
A. There must be a user who made 1994 calls.
B. There must be a user who made exactly 1995 calls.
C. Every user in the city made at least one call during this period.
D. There exist two users who made 1992 and 1993 calls, respectively.
40 D. Let user $A$ be one of the users who make the most calls, with $A$ making $n$ calls, then $n \geqslant 1$ 1993. Among the users $B_{1}, B_{2}, \cdots, B_{n}$ who have calls with $A$, any two users make a different number of calls, but each of $B_{1}, B_{2}, \cdots, B_{n}$ has made at least one call, so the set of call counts for $B_{1}, B_{2}, \cdots, B_{n}$ is $\{1,2,3, \cdots, n\}$.
Since $n \geqslant 1993$, among $B_{1}, B_{2}, \cdots, B_{n}$, there must be $B_{i}, B_{j} \quad(i \neq j, i, j \in\{1$, $2, \cdots, n\}$ ), where $B_{i}$ made 1992 calls and $B_{j}$ made 1993 calls.