Girls have been teasing boys . . . . . 20 minutes. A. since B. for C. from D. in

The correct answer is A. since.

The word “since” is used to indicate a point in time in the past from which something has been happening or continuing. In this case, the girls have been teasing the boys for 20 minutes.

The other options are not correct because they do not indicate a point in time in the past. The word “for” is used to indicate a period of time during which something happens or continues. The word “from” is used to indicate the starting point of something. The word “in” is used to indicate a location or position.

Here are some examples of how the other options could be used in a sentence:

• The girls have been teasing the boys for 20 minutes.
• The girls have been teasing the boys since the beginning of class.
• The girls have been teasing the boys from the other side of the room.
• The girls have been teasing the boys in the hallway.