Is oxygen the only gas _______ helps fire burn?

A. that    B. /      C. which   D. it

Is oxygen the only gas _______ helps fire burn?

A. that      B. /     C. which   D. it

Is oxygen the only gas _______ helps fire burn?

A. that 　　 B. / 　　 C. which 　　 D. it

Is oxygen the only gas _______ helps fire burn?

A. that　　 B. / 　　　 C. which 　　　 D. it

Is oxygen the only gas _______ helps fire burn?

A. that B. / C. which D. it