16-14 As shown in the figure, point $B$ is on line segment $A C$, $M$ is the midpoint of line segment $A B$, $N$ is the midpoint of line segment $A C$, $P$ is the midpoint of $N A$, and $Q$ is the midpoint of $M A$.

Then the ratio $M N: P Q$ is
(A) 1 .
(B) 2 .
(C) 3 .
(D) 4 .
(2nd "Hope Cup" National Mathematics Invitational Competition, 1991)