They need to be certain of several hours of clear air as it will take two hours for an initial reconnaissance team to reach the area where the missing men are.
搜救人员需要确定干净的空气能保持数小时，因为打头阵的侦察队确定被困矿工的位置就需要两个小时。
2
This image was taken in February 2009 by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. It covers an area around.6 miles across.
该照片由美国宇航局火星探测轨道器上的高清晰科学实验成像仪(HRISE)拍摄，摄于2009年2月。
3
During the process of reconnaissance, exploitation, transportation and deposit, a large area around the oil field suffered from serious pollution.