Earthquakes happen without warning(前兆). They can happen any time of a day, at any point during the year. But don't worry because most are so weak that they cannot be felt. Only a few big ones hurt people. However, it's important to know what to do when an earthquake is happening.
   Do NOT go outside. You could get injured from falling glass or parts of buildings. If you are outside, stay away from buildings and power lines (输电线).
   Stay under a desk, table, or other strong furniture（家具）. Hold on to it. Or stay in a corner of the building. Cover your face and head with your arms and cover your mouth with a towel（围巾） or clothing. Stay inside until shaking stops and it is safe to go outside.
   Keep away from windows, pictures or advertisements on the wall or the building, and anything else that could fall and hurt you. Most people get injured by falling things during an earthquake, not by the shaking itself.

   Keep away from a fire. You could fall down and burn yourself on the fire.
   If you are driving when an earthquake happens, stop the car if it's safe. Stay inside your car until the earthquake stops, and don't drive near bridges. Try not to stop by power lines or trees. These could fall and hurt you.

从蝗虫的发育过程看，扑灭蝗虫的最佳时期应在（　　）