$7 \cdot 93$ If $F(n)$ is a function, where $F(1)=F(2)=F(3)=1$, and $F(n+1)=\frac{F(n) \cdot F(n-1)+1}{F(n-2)}$, then $F(6)$ equals
(A) 2 .
(B) 3 .
(C) 7 .
(D) 11 .
(E) 26 .
(21st American High School Mathematics Examination, 1970)