Which of the following early urban societies did not always build major cities in river valleys? a. the huang he valleyb. mesopotamiac. the indus valleyd. mesoamerica   

The correct answer is D. Mesoamerica. 

All of the others are well known for building major cities, thus developing as societies on the river banks of the largest rivers that were of life importance for food production. In Mesoamerica that is not the case, in here the major cities were very often built on high lands, usually on a big, vast, plateaus.

Answer (d) the greenhouse effect is primarily caused by car and trucks;

Intrusion is the best answer and the newest form.