Grote Reber Conducted the First Radio Survey of the Sky

Multiple Choice

Grote Reber conducted the first radio survey of the sky in the 1930s and 1940s with his 9-m-diameter radio telescope.Why did his telescope need to be so large?

A) He needed a large light-collecting area because radio sources are notoriously dim.
B) He needed better angular resolution to identify sources because radio waves are so long.
C) He needed a higher magnification to identify sources because radio sources are quite small.
D) He needed a longer focal length because radio sources are so far away.
E) He needed a shorter focal length because radio sources are so far away.