Which condition must be TRUE for the Congressional Budget Office's assumption about the tax incidence of payroll taxes to be correct?

Question

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The assumption regarding tax incidence applies to the payroll tax only when labor supply is inelastic. Payroll taxes are assumed to be passed on to workers in the form of decreased wages when labor supply is inelastic. If labor supply were elastic, the payroll tax burden would fall mostly on employers, as they would not be able to pass on the tax in the form of lower wages.

Which Condition Must Be TRUE for the Congressional Budget Office's