A:①②③
B:①②④
C:①③④
D:②③④
试管在化学实验中常与其他仪器和药品同时使用(图中其他仪器和药品略去)。下列试管的朝向与实验项目相对应且正确的是( )
(____)A. There is a school in my village.
(____)B. There is a river on the mountain.
(____)C. There're many animals in the forest.
(____)D. There is a tree behind the building.
(____)E. There is a lake near the house.
对于全集U的子集M,N,若M是N的真子集,则下列集合中必为空集的是( )