[单选题]
A convex lens has a f eyf3 zucis7dl dmnl ih4 )fny-10 *8x+ade3,foc(8u yvj aey-7eal 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 $2f$.
