Choose one sentence that has the same meaning as the root one:
The climbers have to use oxygen canisters, which is essential.
A. The climbers should not have used oxygen canisters.
B. The climbers do not know whether they have to use oxygen canisters or not.
C. It is not obligatory for the climbers to use oxygen canisters.
D. It is essential that the climbers have to use oxygen canisters.