Explain each of the following statements using "supply-and-demand diagrams".

a. "Since coronavirus hit the world in early 2020, the price of masks has gradually dropped after a year."

b. "After corona pandemic prevailed the world for more than a year, Brent crude oil price is back at pre-pandemic level (over $60 a barrel as of May 2021)."

The price of cigarettes has increased to around 10 per pack last year in 2020, according to the rise of the tax on it. Please discuss it in terms of market welfare - social cost and benefit. Use the concept of tax revenue and deadweight loss.

(FYI : https://www.francetvinfo.fr/sante/drogue-addictions/lutte-contre-le-tabagisme/paquet-de-cigarette-a-10-euros-qui-touche-quoi_3823105.html

https://www.bfmtv.com/economie/economie-social/en-2020-le-tabac-a-genere-1800-euros-de-taxes-par-fumeur_AV-202104020281.html )