题干
When I walked past the park, I saw some old people _________ Chinese Taiji.
A：do
B：did
C：doing
D：are doing
答案(点此获取答案解析）
C
When I walked past the park, I saw some old people _________ Chinese Taiji.
A：do
B：did
C：doing
D：are doing
cH2O+dCO2配平后a、b、c、d之和为（ ）