{{i}}quEstions 17～20 ArE BAsED on thE Following tAlk. you now hAvE 20 sEConDs to rEAD quEstions 17～20.{{/i}}

ACCorDing to thE spEAkEr, how Do DEvElopErs ContriButE to thE rEDuCtion oF AmphiBiAn populAtionA．By tAking ovEr ponDs.
By ConstruCting sEwErs.
C．By BuilDing DAms on rivErs.
D．By FlooDing mArshEs.