*** Complete the sentences by putting the verbs in the group into the Past Simple, the Past Continuous or the Past Perfect. Write your answers into the table.

perform
a Dr Stephens ____ an experiment while his students were watching him.
b Dr Stephens ____ a lot of experiments before he made an important discovery.
c Two weeks ago Dr Stephens ____ an experiment with the help of some university students.

	Type your answers.
a
b
c