选择题：Please identify the casue and effect of the problem in the f

Please identify the casue and effect of the problem in the following situation.In recent decades, many people have poured into the cities and the cities

have grown so large that now about 50% of the Earth's population lives

in urban areas.

A.The problem is "The cities have grown so large."

B.The cause is "Many people have poured into the cities."

C.The effect is "Now about 50% of the Earth's population lives in urban areas."

D.None of them