Which of the following is not the reason why comp

Which of the following is not the reason why companies have begun to shift their attention on the retention of workforce？

A．the constant improvement of economy in the US.

B．the demand of experienced and well-trained employees.

C．the demonstration of its market strength indicator.

D．the possible lack of workforce in the near future.

请帮忙给出正确答案和分析，谢谢！