Multi-select problem: How does the next paragraph end?

How to stretch to improve posture
Evaluate your posture.
It can be tempting to slump forward at our desk during the day, which can wreak havoc on your neck and shoulders. Your feet should be flat on the floor in front of you.

Available options:
(A). That means any prop you use is called a footrest, and you want to stay centered. On the other hand, balancing means standing at a higher level to improve the flexibility of your spine and neck muscles.
(B). Arms should be straight, legs should be loose, and you should be sitting in the chair. Be conscious of where your feet are placed when you're seated.
(C). You can also get into a serious slouch. It may feel like you should be hunching over or crossing your arms over your chest.
(D). There should be a bit of a gap between the back of your knees and your seat. To keep your back straight, stretch your head up to the ceiling and then tuck your chin in a little bit.
(D).