题干
Only when _________ be possible to sign the papers.
A：does the boss come will it
B：the boss comes will it
C：has the boss come it will
D：the boss comes it will
答案(点此获取答案解析）
B
③白磷 ④18O ⑤ 
⑥红磷 ⑦16O ⑧氯气．