Target heart rate is

a) a heart rate monitor you can purchase at target.

b) heart rate after completing a period of exercise.

c) both 1 and 3

d) desirable minimum heartbeats needed to reach a level of activity and maintain that heart rate necessary to improve your cardiovascular fitness.

for me i have felt invisible when i was clearly struggling,or,i made a great achievement ,and,nobody took to notice me either because they just didn't see it,or,people just turned their back on me.i can see a person being invisible by choice,or,without choice like i did.the people who are invisible by choice don't try to talk to people,don't try to make friends,and,keep to themselves.i haven't been one of those people so i couldn't give you a reason.

hope this and can i get bainliest my dude : d