问题

When I walked past the park, I saw some old people _______ Chinese Taiji.
A:do
B:did
C:doing
D:are doing;
B:did
C:doing
D:are doing;
正确答案是:
同类题型
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The running water makes the stones ______ very smooth.
How many______does a cow have——Four.
There must be somebody in Tom`shome.I`m sure that either Tom or his parents _______ at home now.