Answer:
The constant in the student's investigation is the length of activity time.
In the given question, the student measures her heart rate after each of the activities listed. To determine the impact of different activities on her heart rate, the student needs to keep one variable constant to make valid comparisons. The only option that remains constant throughout the investigation is the length of activity time.
By keeping the length of activity time consistent, the student ensures that she is comparing the effects of different activities on her heart rate under the same duration. This allows for a more accurate assessment of how each activity affects her heart rate, as other factors such as energy expended during the activity or equipment needed may vary between activities.
For example, if the student measures her heart rate after running for 5 minutes and then measures it again after swimming for 10 minutes, the results would not be comparable since the duration of the activities is different. However, if the student measures her heart rate after running for 5 minutes and then after cycling for 5 minutes, she can make a more meaningful comparison because the length of activity time is constant.
In summary, the constant in the student's investigation is the length of activity time. By keeping this variable consistent, the student can accurately compare the effects of different activities on her heart rate.