The increasing order of the rate of HCN addition to compounds

(A) $\mathrm{HCHO}$

(B) ${\mathrm{CH}}_3{\mathrm{COCH}}_3$

(C) ${\mathrm{PhCOCH}}_3\text{, }$and 

(D) $\mathrm{PhCOPh}$ is 

Increase in crowding at the transition state decreases the rate of addition of HCN. 

The increasing order of the rate of HCN addition to compounds D < C < B < A.