多项选择题

Which of the following CRS upgrades are allowed?()

A.CRS 2.2 to CRS 3.5 
B.CRS 3.1 to CRS 4.0 
C.CRS 2.2 to CRS 4.0 
D.CRS 3.5 to CRS 4.0 
E.CRS 4.0 to CRS 3.0 
F.CRS 2.1 to CRS 4.0