植物的光合作用与呼吸作用相平衡之处称为______。
参考答案：光补偿点
目前预防龋病效果最好的糖的代用品是（）。
A.糖精
B.山梨醇
C.木糖醇
D.甘露醇
E.蛋白糖
Where's Tom? His mother ______ him now. [ ]
A. is looking for
B. will look for
C. has looked for
D. looks for
