从后面的yet来看应该用现在完成时,因此是has not been decided 因为同时用两个或更多的疑问词引导的主语从句看作是一个整体,谓语用单数.但如果是两个并列的主语从句作主语,谓语应该用复数.
如：What he has done and why he did it are not quite clear.