Robert Koch and His Lab Workers First Used Agar in Bacteriological

Robert Koch and his lab workers first used agar in bacteriological media.Why did they find this media so much better than previous materials used to grow bacteria?

A) Bacteria,growing on the solidified media,did not degrade or break down the medium.
B) Agar could convert from liquid to solid form,and back again,very easily.
C) The food material that Koch was previously using in the lab was much more expensive than agar.
D) Agar is a great source of nutrients for bacteria.