Which of the following statements is not true?

a.The unemployment rate cannot be zero because the working age population includes all adults such as stay-at-home husbands.

b.The unemployment rate cannot be zero because there is always a mismatch between skills that the employers demand and that people seeking jobs have.

c.The unemployment rate cannot be zero because it takes time for workers to switch jobs.

d.The unemployment rate cannot be zero because people do not always know what jobs are available when they are seeking one.