Why are Carbon-14 and carbon-12 considered to be isotopes?

A. Carbon-14 decays at a faster rate that Carbon-12
B. Carbon-14 is more stable than Carbon-12.
C. Carbon-14 has more neutrons than carbon-12.
D. Carbon-13 is roughly 2 any heavier than Carbon-14.
E. Both atoms have six protons in the nucleus, but have different atomic masses.

The amount of heat required to change liquid water to vapor at its boiling temperature is 2256 kj/kg. the amount of heat required to change liquid mercury to its vapor state at its boiling temperature is 295 kj/kg. one kg of each substance is currently at its boiling point. how will the amount of thermal energy required to change each substance from a liquid to a gas differ?