消化系统和呼吸系统上皮来源于（）
A.外胚层
B.内胚层
C.中胚层
D.外胚层与中胚层
E.内胚层与中胚层
参考答案：B
He has made another wonderful discovery,___ of great importance to science.
A．who I think it
B．which I think it is
C．that I think is
D．which I think is
把一块铁锻打成一定形状的工件以后，它的[ ]
A．质量改变，体积不变
B．形状改变，质量不变
C．质量改变，状态不变
D．形状改变，质量改变
