They also have very thick skin on the soles of their feet, with many more blood vessels than lowland people, so their feet don't freeze.
It is on the Jewish flag from the country of Israel.
Invisible ink: During the Revolutionary War invisible ink usually consisted of a mixture of ferrous sulfate and water.
secret codes: Ciphers and secret codes were used to ensure that the contents of a letter could not be understood if correspondence was captured.
Mask letters: Search Results
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A "Mask Letter" is the name given to a method that the British used during the Revolutionary War to conceal a secret message. This method allowed the secret message to be hidden in plain sight. ... To discover the hidden message, the reader would place a "mask" on top of the letter to reveal the secret message
The answer would be C, as through process of elimination we can determine that both A and B are untrue for this verse, and D would make no sense because Heaven should not be buried it should be shared.