1. Given
$$
\begin{array}{l}
A \cup B \cup C=\{a, b, c, d, e, f\}, \\
A \cap B=\{a, b, c, d\}, c \in A \cap B \cap C .
\end{array}
$$

Then the number of sets $\{A, B, C\}$ that satisfy the conditions is ( ).
(A) 100
(B) 140
(C) 180
(D) 200
-, 1. D.
As shown in Figure 3, the set $A \cup B \cup C$ can be divided into seven mutually exclusive regions, denoted as $I_{1}, I_{2}, \cdots, I_{7}$.

Given that $c \in I_{7}$, the elements $a, b, d$ belong to either $I_{4}$ or $I_{7}$, with a total of 8 possible combinations. The elements $e, f$ belong to one of the regions $I_{1}, I_{2}, I_{3}, I_{5}, I_{6}$, with 5 possible choices each, resulting in 25 possible combinations.
Therefore, the number of sets $\{A, B, C\}$ that meet the conditions is 200.