During thunderstorm formation, a lot of energy builds up in cumulonimbus clouds because some processes that help form the cloud and that occur within the cloud give off energy. What process helps build up energy in cumulonimbus clouds?

Water droplets freeze in the cumulonimbus clouds.
Warm air sinks as the cumulonimbus clouds form.
Warm, moist air mass collides with a cold, dry air mass.
Ice crystals in the upper atmosphere melt.