The Objective of an Astronomical Telescope Has a Focal Length $\times$

Multiple Choice

The objective of an astronomical telescope has a focal length of 900 mm.The telescope has eyepieces of focal lengths 20 mm,15 mm,and 10 mm.What is the highest magnification for this telescope?

A) 920 $\times$
B) 900 $\times$
C) 880 $\times$
D) 90 $\times$
E) 60 $\times$