Lời giải:
Các tập hợp con của $B$ là:
$\left\{a\right\};\left\{b\right\};\left\{c\right\};\left\{1\right\};\left\{2\right\};\left\{\varnothing\right\}$
$\left\{a;b\right\};\left\{a;c\right\};\left\{a;1\right\};\left\{a;2\right\};\left\{b;c\right\};\left\{b;1\right\};\left\{b;2\right\};\left\{c;1\right\};\left\{c;2\right\};\left\{1;2\right\}$
$\left\{a;b;c\right\};\left\{a;b;1\right\};\left\{a;b;2\right\}; \left\{b;c;1\right\};\left\{b;c;2\right\}; \left\{c;1;2\right\};\left\{a;c;1\right\};\left\{a;c;2\right\}; \left\{a;1;2\right\}; \left\{b;1;2\right\}$
$\left\{a;b;c;1\right\};\left\{a;b;c;2\right\}; \left\{b;c;1;2\right\}; \left\{a;c;1;2\right\};\left\{a;b;1;2\right\}$
$\left\{a;b;c;1;2\right\}$