Answers:
B.It helps to convey a sense of expansiveness.
C.to give historical background on the Kiowa route to Rainy Mountain
The Way to the Rainy Mountain is a book by autor N. Scott Momaday. The book tells the journey of Momaday's Kiowa ancestors from their beginnings in the Montana area to their surrender at Fort Sill and subsequent resettlement.
The introduction of this book establishes the setting of the story and provides background historical information on the Kiowa route.
In this passage, the word "billowing" describes the movement of the clouds, and it contributes to the sense of immensity of the sky.