对于变量PRICE，其值为1688，执行以下命令中，能输出字符串$1688的是______。
A. echo '$PRICE'
B. echo "$PRICE"
C. echo `$PRICE`
D. echo \$$PRICE
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After I _______ for an hour, I realized that he was not coming.
A. was waiting
B. had been waiting
C. waited
D. have waited
Scientists concluded that smoking ______ harmful to a person’s health.
A. is
B. be
C. being
D. was
That was the fifth time that he ______ about that touching film in a month.
A. had talked
B. have talked
C. talked
D. was talking
The nurse ______ on a night shift for several weeks.
A. is working
B. works
C. worked
D. has been working
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