In our school library there          a number of books on science, and in these years the number of them          growing larger and larger.

5+____=12          8+____=13        3+____=19

10－____＝3     14－____=4       ____－9＝10

8＋____＝10 ____＋3＝10   10－____＝6      12－____＝2

____＋3＝13  17－____＝12     ____－7＝3      ____－5＝10

人体内食物的化学性消化开始于（    ）