How is it a convex function?

Question

In one of the research paper it is mentioned that the following is a convex function $$f(v_1,\cdots v_N)=\frac{a_1v_1+\cdots +a_Nv_N}{c+\sum_{i=1}^Nd_iv_i}$$ where $a_i,d_i,c$ are some real constants. I do not know how it is a convex function. I have read in Convex Optimization book that such functions are quasi-convex. But is this function also convex?