10 The number of positive integral solutions $(a, b, c, d)$ satisfying $\frac{1}{a}+\frac{1}{b}+\frac{1}{c}+\frac{1}{d}=1$ with the condition that $a<b<c<d$ is
(A) 6
(B) 7
(C) 8
(D) 9
(E) 10
10 Answer: (A)
$$
(a, b, c, d)=(2,3,7,42),(2,3,8,24),(2,3,9,18),(2,3,10,15),(2,4,5,20),(2,4,6,12) \text {. }
$$