表示氢气燃烧热的是（　　）

    Today is October 4th．It's World Animal Day．Gina  and her sister Sally go to the____early．They want to do some things for the____there．In the south of the zoo，Gina finds an elephant．Its____ are kind of dirty．Gina and Sally get some____and clean them．After that they get some food for this elephant．After the elephant eats the food，the two girls take a photo with it．Then they play with it．

    This elephant is____．It can draw．It can play soccer,too．Gina and Sally like it very much．Gina and Sally play with it for a long time．They have a great time in the zoo．

若t>0，那么$\frac{1}{2}$a+$\frac{\mathrm{}}{\mathrm{}}$t与$\frac{a}{2}$的大小关系是（       ）