試題詳解

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(A) Duolingo's mission is philanthropic and the approach is genius.
(B) It also charges for subscription if users want to block the advertisements.
(C) Duolingo is popular for the same reason why Candy Crush and arcade-style video games are successful.
(D) What users don't realize is that the texts they are asked to translate during the learning process are later used by real websites.
(E) Other language learning tools, Rosetta Stone, for example, have historically been only available to those with the disposable income to fund their learning.