Younger people also need to do warm-ups because they have more flexible muscles they have warmer muscles their muscles can also get injured none of the above

I think the answer to this would be :

Younger people also need to do warm-ups because their muscles can also get injured.

There is no person in the world where someone's muscles would not get injured if they do not stretch or do warm-ups. It is the way of life that the muscles can be more stiff if no loosened up and this sometimes results in injuries or sprains.

Iguess it you with your back