For many Americans the automobile is a ________ (necessary).
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Excessive dosage( 剂量 )of this drug can result in ________(injure)to the liver.
“Thank you.”is a polite _______ (express)which is widely used in English.
Though he didn'twin any medals in the last sports meet, he was not ____ (courage).
I wrote him a letter to show my______ (appreciate) of his thoughtfulness.