The most common thing to do on earth, such as walking and eating, can be ________ big threat if not handled(处理)properly in ________ space．

1. A.
   a;/
2. B.
   /;/
3. C.
   a; the
4. D.
   /; the

The most common thing to do on earth，such as walking and eating，can be       big threat if not handled properly in        space.

   A．a；不填          B．不填；不填       C．a；the          D．不填；the