The answer is: [A] "all of these" .
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Note: "ALL" of the the answer choices provided are correct.
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The Mayan calendar was significant for:
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[B]: "[held] the key to deciphering the hieroglyphic tablets found in Palenque."
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[C]: "[measured] time date back to August of 3114 B.C.E."
{Note: "B.C.E." means "Before Common Era". Same thing as "B.C." (before Christ). There has, for quite some time been a "movement" to change "B.C." and "A.D."; to: "B.C.E." ("Before Common Era") and "C.E." (Common Era); respectively. In practice, both are used. Just remember: "B.C.E. = B.C." ; and: "C.E. = A.D."
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[D]: "[included] a "long count" calendar based on 5,200 years cycles."
{Note: The Mayans developed and used a "calendar round" and a "Long Count".}.
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