What Is the Most Important Reason That Astronomers Have Learned

Multiple Choice

What is the most important reason that astronomers have learned more about our planetary system in the last 30-40 years than all of history before then.

A) astronomers today are a lot smarter than astronomers were earlier
B) the Hubble Space Telescope
C) we have been able to send spacecraft to gather information about planets and moons up close
D) radio telescope arrays allow astronomers to make out details on the planets that they have never been able to see before
E) the planets (moving in their slow orbits around the Sun) happen to be closer to the Earth in the last 30 years than at any previous time in human history