通常分为常开型，常闭型和自保持式三种。
Usually divided into normally open, normally closed and self-holding three.
那意味着他们必须亲密合作，但同时也必须对各自保持警惕。
That meant they had to work closely together, but were also able to keep an eye on each other.
那意味着他们必须亲密合作，但同时也必须对各自保持警惕。
That meant they had to work closely together, but were also able to keep an eye on each other.
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