When you do a experiment you are testing the chemicals it has molclues
Answer:
Natasha had <u>been waiting</u> for half an hour before her friend arrived.
Explanation:
The tense we are looking for here is the past perfect continuous. It shows that an action that started in the past continued up until another moment or action in the past. In this particular sentence, we have two actions: Natasha waiting, followed by her friend arriving. As both actions took place in the past, one before the other, we have to use the combination of the past perfect (in this case continuous) and past (simple) tense.
Construction of the past perfect continuous tense: had been + the verb's present participle (root + -ing).
