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从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21．Because of high demand for Type AB blood, supplies of it are usually limited.
A．the; the B．不填；不填 C．不填；the D．a; the
22．In America, a restaurant or bar bill is called a check, a bill.
21．Because of high demand for Type AB blood, supplies of it are usually limited.
A．the; the B．不填；不填 C．不填；the D．a; the
22．In America, a restaurant or bar bill is called a check, a bill.
