Simplify.

a) [[$ \dfrac{a^5 \cdot a^6}{a^{11}} = $]]

b) [[$ \dfrac{b^2 \cdot b^4}{b^6} = $]]

c) [[$ \dfrac{c^5 \cdot c^4}{c^3 \cdot c^6} = $]]

d) [[$ \dfrac{d^5 \cdot d^2}{d^3 \cdot d^4} = $]]