ABC is an equilateral triangle of side 6 cm. If a circle of radius 1 cm is moving inside and along the sides of the triangle, then the locus of the centre of the circle is an equilateral triangle of side is:

A

$$5  cm$$

B

$$(6 - 2\sqrt{3})  cm$$

C

$$4  cm$$

D

$$(3 + \sqrt{3})  cm$$