$24 \cdot 3$ A plane intersects a right circular cylinder of radius 1 to form an ellipse. If the major axis of the ellipse is 50% longer than the minor axis, the length of the major axis is
(A) 1 .
(B) $\frac{3}{2}$.
(C) 2 .
(D) $\frac{9}{4}$.
(E) 3 .
(37th American High School Mathematics Examination, 1986)
[Solution] As shown in the figure, it is easy to know that the minor semi-axis of the ellipse is the radius of the cylinder, which is 1, so its major semi-axis is 1.5.
Therefore, the major axis of the ellipse is 3.
Thus, the answer is $(E)$.