什么叫手信号？
参考答案：用手持信号灯、旗或徒手显示的信号叫手信号。
什么是微生物的污染？
读一读，排一排。
( ) Add water often.
( ) Put the pot in the sun.
( ) Plant the seeds in the soil.
( ) Wait for a flower to grow.
( ) Wait for a sprout.
