可逆反应2SO₂(g)+O₂(g)2SO₃(g)　ΔH<0在一定条件下达到平衡状态，时间为t₁时改变条件。化学反应速率与反应时间关系如图。下列说法中正确的是

A．维持温度、反应体系体积不变，t₁时充入SO₃(g)

B．维持压强不变，t₁时升高反应体系温度

C．维持温度不变，t₁时扩大反应体系体积

D．维持温度、压强不变，t₁时充入SO₃(g)

— Could you tell me            ? 

— Certainly. In half an hour. [ ]

A. when will the train to Chengdu leave

B. when would the train to Chengdu leave

C. when the train to Chengdu leaves

D. when the train to Chengdu left