20. The graph below shows population growth for
paramecia kept under laboratory conditions for
18 days.
120+
Population Size
80-
40+
0
2 4
X
6 8 10 12 14 16 18
Time (days)
Which of the following statements explains what is
happening in the region of the curve labeled "X"?
A. The population's birthrate is zero.
B. The paramecia are in water that is too warm.
C. The paramecia have used up their food supply.
D. The population's birthrate equals the death
rate.
why?