A:①③
B:②③
C:②④
D:①④
Sunday,June 1
Dear Tony,
Now we are having fun at Tian'anmen Square. The weather is fine. There are many people here. We'll visit the Bird's Nest and the Water Cube. In the evening,we'll have Beijing Duck. I love Beijing. I hope you will come to China soon.
Your friend,
Ben
Tony White__
78 Flower Road__
New York,CBE779D,USA
如图所示,两光滑硬杆水平放置,AOB成θ角,在两杆上各套上轻环P、Q,两环用细绳相连,现用恒力F沿OB方向拉环Q,当两环稳定时OB杆对Q环的弹力为( )
已知下列各数:3.14,0.1010010001,0.0123,兀, 27 , −643 ,其中无理数的个数有( )