According to the population pyramid, in 2000 china had about 120 million people in which age-group

50-64

Explanation:

China is the nation with the largest population in the world, and that has been the case throughout all of the written history, and probably even before that. The country had very large birthrates, which was leading to rapid growth to the population. Because of that, there was even a law that restricted the families as to how many children they can have. Because of that law, the population of China started to gradually age, so the percentage of the younger age groups has been declining, while the percentage of the older age groups has been on the rise. This has led to the age group of 50-64 to account for 120 million people in China, a percentage that has grown almost twice compared to several decades prior to that.

heat released by collision processes during accretion, and heat from decay of radioactive elements

explanation: