The ________________ is the “natural” pacemaker of the heart; the node is specialized cells in the top right atrium which produces the electrical impulses that travel down to eventually reach the ventricular muscle allowing the heart to contract.

单选题

Stem cell can produce the body’s ________________.

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问答题

Up to the Second World War， American foreign policy can be described as swinging between ________________ and ________________.

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单选题

– The Auto Show in the City Stadium has been canceled. – Oh, no! ________________!

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单选题

When the food arrives at the entrance to the stomach, it passes through a sphincterlike valve called ________________.

答案解析

问答题

In the private sector, independent schools are also confusingly called ________________.

答案解析