A sample of wastewater has an initial dissolved oxygen (DO) level of 8 mg/L at t=0 and 2 mg/L at t=5 days. The dilution water has DO levels of 9 mg/L at t=0 and 8 mg/L at t=5 days. If 20 mL of wastewater was diluted to 300 mL, calculate the BOD5
1
60 mg/L
2
75 mg/L
3
100 mg/L
4
120 mg/L