Which changing external condition triggers thirst?
a. lack of oxygen
b. lack of water.
c. disease-causing agents
d. change in temperature

i think its the first one

colors do exist. the color of the sky is actually violet; humans percieve the color of the sky as blue.

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.

. the sierra madre fault zone of southern california

. the san andreas fault california