21. An asteroid 40×10⁶ km away appears on a collision course with earth. What's the minimum size for the asteroid that could be resolved with the 2.4-m-diameter diffraction-limited Hubble Space Telescope, using 550-nm reflected sunlight?
(A) 11 m
(B) 110 m
(C) 1.1 km
(D) 11 km
(E) 110 km

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