I need help understanding c) and d) of attached exercise. Thank you in advance!

2) Cost functions Consider the following total cost function for an individual firm: C (q) = 10 + q + if. The rm faces demand curve D(p) = 20 p. a- At what level of output is average cost at its lowest? b- Draw a graph showing rm average cost against quantity. 0- Suppose that there is an industry with two identical rms, with this cost function. What is the industry total cost curve? What is the industry marginal cost curve? d- Suppose these two rms together act in a perfectly competitive manner. What is the price level in the case where these rms act perfectly competitively? What are the prots of the rms given perfectly competitive pricing