在“绿色氧化剂”双氧水(H₂O₂)溶液中加入二氧化锰后。常温下即可迅速分解产生大量的氧气，同时生成水。关于此法制氧气的下列说法错误的是

①二氧化锰是反应物

②二氧化锰是催化剂

③此法制氧气的优点主要是不需加热，且反应速率快

④此法制氧气并不比用高锰酸钾制氧气好

A．①②

B．②③

C．③④

D．①④

—I came to your home yesterday afternoon, but nobody was in.

— Oh, we ________ some shopping in the supermarket.[ ]

A. have done      

B. did  

C. were doing  

D. are doing