—David promised to come round to have supper with us today.

—But he             . I've been alone for half an hour. [ ]

A. won't

B. hadn't

C. hasn't

D. doesn't

催化剂在化学反应前后：①质量变多；②质量不变；③化学性质改变；④化学性质不变；⑤二氧化锰可以作为各种化学反应的催化剂，在上述说法中正确的是[ ]

A.  ① ③

B.  ② ④

C.  ② ④ ⑤

D.  ② ③