鸭瘟的特征性临床症状是（）
A.咳嗽，呼吸困难
B.体温升离，关节肿大
C.关节肿胀，行走困难
D.精神沉郁，食欲减少
参考答案：C
________ and short of breath, Andy and Ruby were the first to reach the top of Mount Tai. (2009浙江卷)
A．To be tried
B．Tired
C．Tiring
D．Being tired
如图所示四种简单机械中，属于费力杠杆的是[ ]
A．
B．
C．
D．
