海豚用它的鳍在水中优雅地游动。

The dolphin swims gracefully using its ___flipper.

火箭的副翼帮助它在飞行中保持稳定。

The rocket's ___ailerons help stabilize it during flight.

发动机散热器的铝制鳍片需要定期清洁。

The aluminum ___fins on the engine radiator need regular cleaning.