[单选题]
A convex lens has a y)iv8jhylw6j; 53woqny1n/g.yjb8u hj ofye 9p 2ht5ykzbwl4tg4 y86yp55 9focal length $f$. An object is placed at $f$ on the axis. The image formed is located
A. at infinity.
B. between $2f$ and infinity.
C. at $2f$.
D. between $f$ and $2f$.
