Ⅰ.用适当的介、副词填空
1．You have three layers ________ skin which act ______ a barrier ________ disease，poisons and the sun’s harmful rays.
2．First aid is a very important first step ________ the treatment ________ burns.
3．You can get burned ________ a variety ________ things：hot liquids，steam，fire，radiation，the sun，electricity and chemicals.
