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

How to sit at a computer
Sit using ideal posture.
Many office and personal desk chairs have adjustable backs, seats, and even lumbar support. Since the type of chair that you use will vary, try to keep the following criteria in mind :
Your upper legs should be flat against the bottom of the chair.

Choose your answer from: (a). Your lower legs should form a 90 degree angle at the knees. Your feet should be at a flat 90 degree angle to your lower legs.; (b). Your knees should be straight across the top of the seat. Your hands should be on the armrests or on the table next to the chair.; (c). Your lower legs should be straight and slightly curved, indicating that your are sitting at optimal posture. Your feet should be naturally slightly raised on the armrests and your toes should be touching one another.; (d). Your toes should be pointed and your fingers should be loosely bent-this will keep your calves in contact with the table. Your arms should be near your sides and your hands in front of you-don't overlap them or let them rest on the sides of the chair.;
(a).