Bottleneck effect occurs when a population suddenly gets much smaller.
Severe droughts have caused much of the rural population to urbanize.
Severe droughts may cause a bottleneck effect.
Severe droughts may cause 
A: evolution
B: population growth
C: forest is destroyed
D: species variation
E: major threat to health
F: Dehydration
G: the ground shaking
H: a bottleneck effect