—Let me help you , Tom !

—Thank you . I can do it. Here’s         to hold all these things .

A．a big enough case

B．an enough big case

C．a case enough big

D．a case big enough

目前工业上用乙烯制乙醛的过程分为三步进行：

①CH₂=CH₂＋PdCl₂＋H₂O → CH₃CHO＋2HCl＋Pd

②Pd＋2CuCl₂ = PdCl₂＋2CuCl

③2CuCl＋1/2O₂＋2HCl=2CuCl₂＋H₂O

对于整个生产过程，下列叙述正确的是

A．CH₂=CH₂是还原剂，PdCl₂是氧化剂

B．CuCl是还原剂，O₂是氧化剂

C．乙烯是还原剂，O₂是氧化剂

D．Pd和CuCl都是催化剂