The Slenderness Ratio for a Cavity Wall with 102mm Thick

The slenderness ratio for a cavity wall with 102mm thick external brickwork, 75mm cavity, 50mm partial cavity fill, 125mm thick internal concrete blockwork and 12.5mm plaster finish and having an effective height of 2.4m is

A) 7.95
B) 10.57
C) 11.45
D) 11.92
E) 15.86