Answer: [B]: " spore ".
Step-by-step explanation:
" A spore is a single cell surrounded by a protective shield."
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Choice: [D]: "carbon dioxide" — is not a "single cell"—but, to the contrary, is a gas that plants utilize during cell respiration
Choices: [A]: petal ; and:
[C]: stamen ;
are structures of an angiosperm ["flowering plant"]—specifically, the "flower"—that are "multicellular."
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The spore is a single cell and type of "seed" that is protected by a "shield" to ensure the viability of the seed for plant reproduction.
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