Moody’s Investors Service has placed its Aa1 rating on the Massachusetts Bay Transportation Authority’s senior sales tax bonds on review for possible downgrade, affecting roughly $3.8 billion in debt.

Moody’s cited lack of sales-tax growth. It said the MBTA, which oversees Greater Boston’s transportation system, should implement a debt-service improvement plan and mitigate its exposure to variable-rate debt to avoid a downgrade.

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