Tom was playing football on the road. A car was coming, Jack called to Tom, "Look out!". Here "look out"

means ______. [ ]

A. 向外看

B. 当心

C. 向前看

D. 抬头看

“低碳环保”是当今世界的主题，太阳能已被人们广泛利用．太阳是利用原子核发生________（填“裂变”或“聚变”）释放出能量，太阳能属于________（填“可”或“不可”）再生能源．如今通信技术也在向环保靠近，如3G手机已经广泛进入人们的生活，它是由________波传递信息，这种波在真空中的传播速度约为________m/s.