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Terminology is always a challenge imo, i think your teacher's explanation about the usage of "motion" in that sentence is quite satisfied bc "movement" does not have the same meaning as "gesture", while "motion" does, clear enough.
For your question,how about stating your explanation of "movement" usage here, the meaning of "moving his hand" is somehow not "gesture"
Even in science, terminology agreement is arguable also, but we have equations and numbers which make it less arguable. The basis of language/meaning is never clear.
It is ''odd'' to say in common: with the single motion of his hand. I prefer ''movement'' in this case. Motion is used primely to describe law of motion, or an action of something unanimately.
đọc chả hiểu gì cả

[spoil]
có cách nào làm tốt phần pronunciation ko mình làm test phần này sai rất nhiều
.It is high time i went home
câu này sao đáp án lại là went đc nhỉ
đáp án có go / am going / will go / went[/spoil]
just why not one plus one is ******* two 
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