$154 A B C D-A^{\prime} B^{\prime} C^{\prime} D^{\prime}$ is a parallelepiped, $P, Q, R$ are on the edges $A A^{\prime}, B B^{\prime}, C C^{\prime}$ respectively, and $\frac{P A}{P A^{\prime}}=m, \frac{B Q}{Q B^{\prime}}=n, \frac{C R}{R C^{\prime}}=l$, the plane through $P, Q, R$ intersects the edge $D D^{\prime}$ at point $S$, then $\frac{D S}{D D^{\prime}}$ equals
A. $\frac{l}{l+1}+\frac{m}{m+1}-\frac{n}{n+1}$
B. $\frac{1}{l+1}+\frac{1}{m+1}-\frac{1}{n+1}$
C. $l+m-n$
D. None of the above answers
154 D. $S P / / R Q$. So $\frac{S D^{\prime}}{P A}=\frac{R C^{\prime}}{Q B^{\prime}}$,
$$
\frac{S D^{\prime}}{D D^{\prime}}=\frac{P A^{\prime} \times R C^{\prime}}{D D^{\prime} \times Q B^{\prime}}=\frac{\frac{1}{m+1} \times \frac{1}{l+1}}{\frac{1}{n+1}}=\frac{n+1}{(m+1)(l+1)} .
$$