He has worked in the city _______ three years ago while his wife has been here______only one month.

A．since , for

B．since, since

C．for, since

D．for, since

某种生物孢子的直径为0.00063m，用科学记数法表示为[ ]

A．0.63×10^﹣3m

B．6.3×10^﹣4m

C．6.3×10^﹣3m

D．63×10^﹣5m