What makes astronomers think that impact rates for the moon must have been higher earlier than 3.8 billion years ago?

There is strong evidence indicating that 3.8 billion years ago there was a higher impact rate. This deduction starts from comparing the number of craters in the lunar highlands with those of the Mary. If this comparison is made, it will be observed that there are 10 times more craters in the highlands than in a similar area of Mary. It should be borne in mind that through radioactive dating processes the samples indicate that there is a slightly greater antiquity in the highlands than those of Maria. This allows us to deduce that if the impact rates had been constant, the highlands would have been 10 times older. They would have to be formed 38 billion years ago, long before the universe itself began.

Therefore one of the most obvious reasons is there are ten times more craters on the older highlands than the Younger Maria.

d. air pressure decreases as altitude increases.