A、BECAusE it wAs DiFFiCult to CollECt.
B、BECAusE it wAs DiFFiCult to gEt riD oF.
C．BECAusE it CAmE in A vAriEty oF Forms.
D．BECAusE it tEnDED to BE ABsorBED By physiCAl oBjECts.