i want full answer if you can please. it will be helpful. However I can understand that according to your policy sometimes you can only answer 1 part. so if you gone answer only one part so please answer part a.

7. Extra credit (cach is 5 points) (a) Why did the price of oil (WTI) became negative on April 20, 2020? (b) In 6 (e), if B/P and ROE have correlation of 60%, how to overcome the collinearity issue in a multi-factor model