2. As shown in the figure, $A B / / E F / / C D$, it is known that $A B=20, C D=80, B C=100$. Then the value of $E F$ is
(A) 10 ;
(B) 12 ;
(C) 16 ;
(D) 18 .
2. (C)

Solution By the intercept theorem, we get
$$
\begin{array}{l}
\frac{E F}{80}=\frac{B F}{100}, \\
\frac{E F}{20}=\frac{F C}{100} .
\end{array}
$$
(1) + (2), we get $\frac{E F}{80}+\frac{E F}{20}=\frac{B F+F C}{100}=1$.
$\therefore \quad 5 E F=80, E F=16$.