16. I have been searching for this information for two hours, but I can't find anything. We use the present perfect continuous tense in this sentence because it describes an action that started in the past (two hours ago) and is still ongoing (not finding the information). This tense is used for actions that started in the past and have a connection to the present. 17. How many books have you read so far? We use the present perfect tense in this sentence because it describes an action (reading books) that started in the past and has a connection to the present. The word "so far" indicates that the action is ongoing or has just been completed. 18. Mary has never bought anything from an eco-bazaar before. We use the present perfect tense in this sentence to express an action (buying) that was completed at an unspecified time before now. The word "never" indicates that the action has never happened before. 19. She has tried the new Italian seafood restaurant that opened last week yet? We use the present perfect tense in this sentence to describe an action (trying the restaurant) that was completed at an unspecified time before now. The word "yet" indicates that the action is expected to happen or has recently happened. 20. We have been discussing this issue for the last three hours! Let's go and have lunch. We use the present perfect continuous tense in this sentence because it describes an action (discussing) that started in the past (three hours ago) and is still ongoing. The word "for" indicates the duration of the action. In summary, the present perfect tense is used to describe actions that started in the past and have a connection to the present, while the present perfect continuous tense is used to describe ongoing or recently completed actions that started in the past.