在SQL中,可以用谓词Exists来测试一个集合是否( )。
A.有相同元组存在
B.为空集合
C.有相同分量存在
D.为非空集合
参考答案:D
The fire started in the basement and quickly____________ to the first floor
where it destroyed all the______________ in the language lab.
A.was spreaded;furniture and equipments
B.was spreaded ;furniture and equipment
C.spread;furnitures and equipments
D.spread ;furniture and equipment
活塞式飞机着陆滑跑的减速力主要是()
A.放襟翼产生的空气阻力
B.刹车时增大的地面摩擦力
C.发动机的反推力