Following are groups of three​ phrases, which group states three ways to improve your​ concentration?

a)Stretch before​ studying, take breaks every 15​ minutes, lie down while reading
b)Keep your cell phone​ on, sign on to​ IM, check your​ e-mail regularly
c)Set up a study​ space, preview your​ materials, create a routine
d)Study in the most comfortable​ chair, listen to​ music, have a snack near by