危险因素
参考答案：危险因素：那些能使人群发病率升高的内外环境因素称为危险因素，常被泛称为病因。（3分）
While you are having an exam, you must write as ________ as you can.
A．careful
B．carefully
C．more carefully
D．most careful
简述土壤湿度的调节与控制方法。
