If the typical correlation between stock returns is $0\mathrm{.}15$ and the typical variance of each

stock's return is $50\%,$ then:

A$.90\%$ of the unique risk can be eliminated if we hold over $100$ stocks

B$.$ Market risk $($aka non diversifiable or systemic risk$)$ is about $20\%$

C$.$ Most of the unique risk remains if we hold $10$ or more stocks

D$.$ Most of the market risk can be eliminated by holding $20$ stocks

Correct: B

Pls explain in detailed, thanks!