Longitudinal studies are done very quickly

Probably because you wake up early. even when i get a good nights sleep i still feel tired if i wake up early. or maybe you just need coffee.

Afew buddies come over to your house to hang out. they notice you're struggling in school, perhaps you have to rely on to pass your class, but you can't absorb the knowledge. they could positively reinforce getting a tutor and working with you, dedicating more time to the areas of struggle and less on hanging out to better yourself.secondly, say you're the victim of bullying. you're afraid to tell an adult because of possible repercussions from the bully himself. your friends encouraging you and pushing you towards telling an adult for your safety is another example. 

Target heart rate is the answer

Cwould be the correct answer i believe