单项选择题
An example of an anchor which has a stock is a（）.
A.Bruce anchor B.Dunn anchor C.Hook anchor D.Danforth anchor
单项选择题 An example of a grade C product is（）.
单项选择题 An example of a grade B product is（）.
单项选择题 An entry of water，gas，oil，or other formation fluid into the wellbore is called a（）.
