题目

---Look! Here comes the bus and I have to go now. Thank you for coming to see me off.    --- Goodbye, _______.    A. congratulations    B. cheer up    C. all the best        D. go ahead 答案：C下列性质中，属于氢气的化学性质的是（ ）A．无色气体B．比空气轻C．难溶于水D．可燃性