Weighing the CO2 in soft drink

National Science Quiz 2021

Question:
Which weighs more, an unopened can of soft drink, or an open can of flat soft drink?
A. The unopened can
B. The open can
C. They weigh the same

Background: We all know that Albert Einstein invented carbonated beverages by splitting the beer atom in Tasmania. Ok, that may not be entirely true, but it is true that carbonated beverages are some of the most popular drinks in the world, and can be found everywhere on earth --- and even above the earth! Astronauts aren't allowed to have soft drink, but if they did, then because everything is weightless, the bubbles don't rise to the top of the drink, and so it never goes flat! On earth, of course after you open the can the carbon dioxide will all eventually escape so it does go flat.