扦插苗床土壤应经常保持湿润，苗床的相对湿度保持在（）以上为宜。
A.70%
B.80%
C.90%
D.100%
参考答案：C
Many students signed up for the _____ race in the sports meeting to be held next week.w
A．800-meter-long
B．800-meters- long
C．800 meter length
D．800 meters length
血红素合成调节的限速酶是
A．ALA合酶B．ALA脱水酶C．亚铁螯合酶D．HMGCoA合成酶
