英语问题The skin is the largest organ of your body _____ of thre

英语问题
The skin is the largest organ of your body _____ of three layers,______ are essential to _____ your body body against diseases,poisons and the sun's harmful rays of radiations.
A.consisting,they,protecting
B.consisting,which,protecting
C.made up of,they,protect
D.consisted,they,protecting
为什么选A