1. exercise
2. range of motion
3. muscle myofibrils
4. lactic acid
5. poor posture
6. varicose veins
7. antagonistic muscles
I think it's C because he acts suitable according to his age hut the little sister behaves selective.
Hope it helps!
#MissionExam001
The answer is no you can not feel spasm twinges when sperm fertilize the eggs