国债在到期之前并不能被赎回，也没有赎回条款。
Treasury securities cannot be redeemed before maturity and do not have call provisions.
RowsInSection基于section参数传递进来的分段索引来返回条目的数量。
RowsInSection should return the number of items in whatever section it passes in via the section parameter.
但是，一些在1985年前发行的国债有赎回条款，所以你需要注意你是否在二级市场上买了这些债券。
Some Treasury bonds issued before 1985 did have call provisions, however, so you need to watch out if you buy these bonds in the secondary market.
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