What Is It About the Sun's Corona That Astronomers Don't

What is it about the Sun's corona that astronomers don't understand?

A) No one knows why that part of the Sun's atmosphere does not drift away into space.
B) During total solar eclipses, the corona sometimes disappears from view.
C) The corona seems to absorb 2/3 of the neutrinos that pass through it.
D) The corona is much hotter than layers of the Sun that are closer to the solar interior.
E) The Sun's corona extends to the outer reaches of the solar system.