一辆汽车从北京驶往天津，每小时行驶150千米，行a小时后，距天津还有50千米。从北京到天津共有（ ）千米。
150a+50
According to the companies' websites, the number of active QQ users in China has reached 1 billion, while Qihoo 360 also claims _______ at least 300 million users before June 2010.[ ]
A. to have
B. to have had
C. having
D. having had
活塞杆填料函具有密封（）的作用，不使其漏人曲轴箱中。
A．扫气空气
B．燃气
C．气缸下部空间气体
D．废气
