What is responsible for much of the erosion of Earth's surface? ...

[stevenposner]

What is responsible for much of the erosion of Earth's surface?
 
  a. lava flows
 b. solar wind
 c. impact craters
  d. convection
 e. water

Question 2

What observations demonstrate the existence of an interstellar medium?
 
  What will be an ideal response?

[yasmina]

Answer to Question 1

e

Answer to Question 2

1) We can see the three different types of nebulae with the aid of optical telescopes. 2) As starlight travels through the ISM, gas atoms can absorb specific wavelengths and form interstellar absorption lines in the spectra of those stars. Astronomers can distinguish interstellar absorption lines from stellar absorption lines in a number of ways. 3) You expect from Kirchhoff's second law (Chapter 7) that an excited low-density gas viewed against an empty or colder background should produce an emission spectrum, and the ISM does exactly that.

[stevenposner]

Great answer, keep it coming

[sultana.d]

Excellent