—______ does your cousin usually go to work on foot?—He says it's good for his health.
A:Where
B:When
C:Why
D:How
已知 a−b=3,ab=2 ,求下列各式的值。
①适当提高二氧化碳浓度 ②提高氧气浓度
③适当延长光照时间 ④形成高温环境 ⑤人工形成昼夜温差较大的条件.