a. In summer, the temperature is optimum for the growth of Lactobacillus bacterium which converts milk into curd than in winter. So these bacteria multiply very fast causing milk to turn into curd rapidly and produce more lactic acid making curd sour faster.

b. Virus show characteristics of both living and non-living. When they are outside the body of the host they behave like non-living. But once they enter into a body of host whether plant, animal or bacteria, they start multiplying and behave like living beings. So viruses are neither living nor non-living.