根据《地处规》第4.2.7条及《地规》第5.3.5条

$$ l / b = \frac {3 . 0}{0 . 6} = 5. 0, z _ {1} / b = \frac {0 . 6}{0 . 6} = 1. 0, \overline {{{{\alpha_ {1}}}}} = 0. 2 3 5 3 \times 4 = 0. 9 4 1 2 $$

$$ l / b = \frac {3 . 0}{0 . 6} = 5. 0, z _ {2} / b = \frac {3 . 0}{0 . 6} = 5. 0, \overline {{\alpha_ {2}}} = 0. 1 3 2 5 \times 4 = 0. 5 3 0 0 $$

$$ s = 1. 0 \times 9 0 \times \left(\frac {0 . 6 \times 0 . 9 4 0 8}{6} + \frac {3 \times 0 . 5 3 0 0 - 0 . 6 \times 0 . 9 4 0 8}{2}\right) = 5 4. 6 (\mathrm{mm}) $$

因此选（B）。