男,15岁。半年前因右肱骨下端骨肉瘤行截肢手术,半个月前出现右胸痛、咳嗽;X线片示右肺上叶转移性肿瘤。肿瘤转移的途径可能是()
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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