A cylindrical blind riser with diameter d and height h, is placed on the top of the mould cavity of a closed type sand mould as shown in the figure. If the riser is of constant volume, then the rate of solidification in the riser is the least when the ratio h : d is

• A. 1 : 2
• B. 2 : 1
• C. 1 : 4
• D. 4 : 1