我国的四大牧区分别是__________、__________、__________、__________。
内蒙古牧区；新疆牧区；青海牧区；西藏牧区
In order to keep our students safe, strangers ________before entering the school yard.
A. will check B. will be checked C. checked
在熔化过程中，______有一定的熔点，______没有一定的熔点．
