Assertion: If the temperature of a semiconductor is increased then its resistance decreases.

Reason: The energy gap between conduction band and valence band is very small. 

(a) In semiconductors the energy gap between conduction band and valence band is small

($\approx$1 eV). Due to temperature rise, electron in the valence band gain thermal energy and may jumpy across the small energy gap, (to the conduction band). Thus conductivity increases and hence resistance decreases.