Official websites use .gov
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.gov website belongs to an official
government organization in the United States.
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A lock (
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connected to the .gov website. Share sensitive
information only on official, secure websites.
(sB0-1 represents the first stage in the surface layer; sB0-2, the first stage in the mid layer; sB0-3, the first stage in the deep layer; sB18-1, the last stage in the surface layer; sB18-2, the last stage in the mid layer; and sB18-3, the last stage in the deep layer).
