19.
(A) problem
(B) fact
(C) reason
(D) likelihood

15. (A) about (B) readily (C) likely (D) concerned 

16. (A) in two weeks (B) this past week (C) since two weeks ago (D) for over the two weeks

17. (A) Two-third of the adult (B) Most of adults (C) Two-thirds of adults (D) The most of the adults

18. (A) more than (B) as (C) for (D) or

20. (A) considerable influence (B) preposterous suggestion (C) prophetic wisdom (D) irrevocable decision

複選題21 (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.

複選題22 (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.

複選題23 (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.

複選題24 (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.

複選題25 (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.