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 think about how to calculate NPV. the discount rate for a project is WACC, if wacc decrease, then some projects with -tive NPV previously might turn to be profitable which is worth investing. is this answer your 1st question?

next, bond yield is inversely related to its price. when bond price decrease, the yield thus increase which in turn increase the wacc. according to above explanation, the # of +tive NPV projects decrease.