Why did Calgary Zoo have to send two giant pandas back to China?

A、Canadahaveasuitableenvironmentforbamboogrowth
B、Duetopandemic-relatedchangesinitsexportlawsandquarantinemethods
C、TheyhavetobesentbacktoChinaforcuringillness
D、Thezoocouldn'tprovideenoughfreshbambooforpandas