1. There are three types of school supplies: pencils, exercise books, and ballpoint pens. If you buy 3 pencils, 7 exercise books, and 1 ballpoint pen, it costs 3.15 yuan; if you buy 4 pencils, 10 exercise books, and 1 ballpoint pen, it costs 4.2 yuan. If you buy 1 pencil, 1 exercise book, and 1 ballpoint pen, it will cost ( ) yuan.
(A) 1.2
(B) 1.05
(C) 0.95
(D) 0.9
-1. (B).
Let the cost of buying one pencil, one exercise book, and one ballpoint pen be $x$ yuan, $y$ yuan, and $z$ yuan, respectively. According to the problem, we have
$$
\left\{\begin{array}{l}
3 x+7 y+z=3.15 \\
4 x+10 y+z=4.20 .
\end{array}\right.
$$

Solving the system of equations for $(x+3 y)$ and $(x+y+z)$, we get
$$
x+y+z=1.05 .
$$