GMAT Critical Reasoning (CR) | Seven countries signed a treaty binding each of them to

"with the actions of each conditional on simultaneous action taken by the other countries"

I didn't quite understand the stem. Are the actions parallel or in series? I would understand that simultaneous = at the same time. If it otherwise means that one country HAS to wait for another country's notification, then C would make sense to me.

Could any one help me out?

Thanks!

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