Choose the best answer.

Scientists have been studying different ecologies ______ the 20^th century.

A. accept  

B. refuse  

C. threaten  

D. study

A. attract

B. attracted

C. attracting

D. to attract

A. Tourists can dive in the coral reef at any time.

B. If tourists stay in the water, they can damage the coral reef.

C. If anyone swims in the water, they will be punished.

D. If anyone damages the coral reef, they will be punished.

A. animals  

B. vegetation  

C. fish  

D. insects

A. learn

B. learnt

C. are learning

D. have been learning

A. Invasive species can be found everywhere.

B. Zebra mussels are being raised in farms.

C. Visitors mustn’t try any leisure activities in the water.

D. Visitors can swim in this lake.