PLS HELP Which reactions are the reverse of one another?

condensation and addition

addition and elimination

elimination and condensation

substitution and addition

Harvey kept a balloon with a volume of 348 milliliters at 25.0˚c inside a freezer for a night.  when he took it out, its new volume was 322 milliliters, but its pressure was the same.  if the final temperature of the balloon is the same as the freezer’s, what is the temperature of the freezer? the temperature of the freezer is  kelvins.

What’s the scientific notation for the number 6,840,000,000

What mass of natural gas (ch4) must you burn to emit 276 kj of heat?