慢速英语:越来越多美国人愿意接种疫苗

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challenge –n. a difficult task or problem; something that is hard to do

probably –adv. something that is likely to happen

definitely –adv. without doubt; in a way that is sure or clear

dose – n. the amount of a medicine, drug or vitamin that is taken at one time

confidence –n. a feeling of being sure that something will happen or that something is true

conversation –n. an informal talk involving two or a small group of people

authorized –n. to give legal or official approval