求翻译：In addition, following changes to rule 2a-7 (the regulatory framework governing registered money-market funds), portfolio managers are required to hold a minimum of 10% in overnight securities and 30% in securities that mature in seven days or less. With an average WAM of 40 days--which is well below historical WAMs--we believe our rated money-market funds are currently well positioned against potential declines in their NAVs as a result of rising interest rates.是什么意思？

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