49 What does the author suggest about SL learning?
(A) Too much evidence shows that the critical period theory is valid.
(B) Through instruction and practice, adult SL learners can acquire good pronunciation.
(C) Second language learning is biologically determined.
(D) SL learners who sound like native speakers are totally nonexistent.

The question of how people learn a second language is one that has received公認的；承認 a great deal of scientific attention.

Research has offered evidence that has been used to support a number of conflicting不一致的；相矛盾的 claims about second language (SL) learning.

However, there is one  fundamenta基礎的，基本的 observation觀察 that is less open to dispute（尤指勞資雙方或相鄰兩國之間的）爭執 than others.

If success in adult second language learning is measured by how close the learner comes to the level of a native speaker, it is possible and quite common for adults to achieve a high degree of success in learning SL grammar and vocabulary.

Strangely enough, the same degree of success does not seem to be attainable可以達到 in SL phonology音韻學；語音體系; adult SL learners who sound like native speakers are extremely rare, perhaps even nonexistent不存在. How have scientists attempted to account for the remarkable inability無能 of adult second language learners to acquire取得，獲得 a native like pronunciation in the second language?

One likely  explanation解釋；說明 is to be found in the so-called critical period theory. proponents提倡者，宣導者 of this theory argue that the acquisition獲得, 收購品 of native-speaker pronunciation in any language is biologically possible only until about the age of twelve.

But it must be emphasized that there is too much counter evidence for such a conclusion結局；結尾；結果 to be valid確鑿的；合理的；有根據; there are too many cases of adults who acquire adequate足夠的；合格 or good pronunciation in a second language through instruction命令, 指示 and practice. While they will never be mistaken for native speakers, their efforts are rewarded報答；報償；酬謝 by speech that is perfectly comprehensible可理解.