从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21．More than five doctors and rescuers have got to Yushu County up to now.
A．thousand B．thousands C．thousand of D．thousands of
22．When I got New York, I had to attend a school for students first language was not English.
A．who B．which C．whose D．where
23．--- Did you telephone Uncle Sam our plan for the Christmas holidays on his farm?
--- Yes, he was happy about our arrival, and he eagerly asked when .
A．has we arrived B．we had arrived
C．were we arriving D．we were arriving
