19. Learning is served by collaboration between Ms. Robinson and her students and among her students. Which theoretical concept facilitates such learning in Ms. Robinson’s class?
(A) ELF (English as a Lingua Franca)
(B) ZPD (zone of proximal development)
(C) LEP (limited English proficiency)
(D) LAD (language acquisition device)

The zone of proximal development refers to the difference between what a learner can do without help and what he or she can achieve with guidance and encouragement from a skilled partner. Thus, the term “proximal” refers to those skills that the learner is “close” to mastering.

The Language Acquisition Device (LAD) is a controversial claim from language acquisition research proposed by Noam Chomsky in the 1960s. The LAD concept is a purported instinctive mental capacity which enables an infant to acquire and produce language. 

The term English as a lingua franca (ELF) refers to the teaching, learning, and use of English as a common means of communication (or contact language) for speakers of different native languages.

British linguist Jennifer Jenkins points out that ELF is not a new phenomenon. English, she says, "has served as a lingua franca in the past, and continues to do so nowadays, in many of the countries that were colonized by the British from the late sixteenth century on (often known collectively as the Outer Circle following Kachru 1985), such as India and Singapore. ... What is new about ELF, however, is the extent of its reach," (Jenkins 2013).

語言學家杭士基(N. Chomsky)提出語言習得裝置的理論，來說明其語言天賦的觀點。他認為人類的認知結構中，存有一種與生俱來的語言獲取機制(Language Acquisition Device；LAD)，使人們可以不需經過刻意教導，只要有適當語言刺激，就能吸收語言及其中的文法規則，獲得語言。
其又將語言分為兩個層次：
一、語言能力(linguistic competence)：指的是靠語言獲取機制(LAD)所得的語言能力
二、語言表現(language performance)：以語言能力為基礎在環境中學習而得的語言。