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. 不填            C. which           D. it