Answer:
I was wrong, the answer is B for all future readers
Cob has been used since the 17th century as a building material. It is made by mixing dry sand, sifted clay and water. A cob house is a house that is made from this material. A benefit of using this material, is that it is sustainable and only uses natural products.
I guess i have to fill in space-His sister-Apex
The adverb " very " modifies the adjective " useful ".
