3 C. The 1991st positive odd number is
$$
\begin{array}{c}
2 \times 1991-1=3981 \\
3969=63^{2}<3981<65^{2}=4225 \\
3981+2 \times \frac{63+1}{2}=4045
\end{array}
$$
3 After removing all the perfect squares from the sequence of positive odd numbers, the 1991st number is
A. 4013
B. 4007
C. 4045
D. 4225