To show an action that started in the past and that is going on in the present, use the a. future perfect tense, so they will have studied for several hours. b. past perfect tense, so they had studied for several hours. c. present tense, so they are studying for several hours. d. present perfect tense, so they have been studying for several hours.

Hello there

To show an action that started in the past and that is going on in the present, use the:

present perfect tense, so They have been studying for several hours.

Explanation: We use the present perfect tense for something that started in the past and continues in the present. It marks a relationship between something that has already started in the past, but it continues taking place in the time the speaker is talking, in the present.

The future perfect would not work because it is used for talking about an action that will be completed between now and some point in the future

The present tense would not work either because it is only stablishing an underway relationship in the present.

Finally, the past perfect indicates that the action already finished and it does not take place in the present anymore.

Hope this helps.

the direction of the magnetic field produced by an electric current will change when the direction of the electric current changes. for example : if a current is flowing in a straight length of wire, the magnetic field is described by a concentric circle (using right-hand rule).

the central idea is basically saying that juveniles that have committed a crime will not have the same consequences as the   adults that have. they will have a softer punishment than what the adults have that is harsh in some ways.