at zero years, total birds = 20
After 10 years, total birds = 20 x 2 = 40
After 20 years, total birds = 40 x 2 = 80
and it continues in that format
so , looking at the graphs,
graph 1 is wrong because total birds in there after 20 years is 40
graph 2 is wrong because the number of birds is decreasing instead of increasing
graph 3 is wrong because total birds after 20 years is 20
graph 4 is correct beacuse total birds after 20 years, tracing it up is 80