How do air masses cause weather changes?

Air masses have the properties of the land below it. Because of this, different temperature air masses colliding can cause high or low pressure. High pressure usually causes cool sunny days, low pressure usually causes warm cloudy/rainy days.

Ithink i might know wait one second let me serve

It's either  a. chiang kai-shek and the nationalists turned against the communists and killed hundreds of them. orb. hundreds of peasants were killed in shanghai during the period of warlord rule.  if we take into account that there was warlord rule at the time, i think that it's b (not entirely sure though)just my best thoughts.