To determine the carrying capacity for rotifers in the absence of water fleas, we can look at the data provided for the number of rotifers alone over the days.
From the information:
- Day 0: 0 rotifers
- Day 2: 0 rotifers
- Day 4: 300 rotifers
- Day 6: 500 rotifers
- Day 8: 800 rotifers
- Day 10: 1700 rotifers
- Day 12: 3000 rotifers
- Day 14: 4000 rotifers
- Day 16: 4200 rotifers
- Day 18: 4200 rotifers
The maximum number of rotifers recorded in the absence of water fleas is 4200, which occurs on Day 16 and Day 18.
Thus, the flask's carrying capacity for rotifers in the absence of water fleas is 4,200.