【題目】The three layers of skin act as a barrier against diseases, poisons and the sun's (射線).

【答案】rays
【解析】rays.考查名詞。句意：皮膚的三層是抵抗疾病、毒物和太陽射線的屏障。“射線”是可數(shù)名詞要用復數(shù)，故填rays.

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