Определите, в каком времени стоят предложения.
18) They have carried out a number of important experiment in this laboratory.
19) Livingstone explored Central Africa in the 19th century.
20) By the middle of autumn we had planted all the trees.
21) They will stage this play at the beginning of next season.
22) They have forgotten the story.
23) Has anybody explained the rules of the game to you?