Poli(vinyl clorua), viết tẳt là PVC, được điều chế từ vinyl clorua  CH 2  = CH - Cl. Để thu được 1 tấn PVC cần bao nhiêu tấn vinyl clorua, giả thiết hiệu suất phản ứng là 90% ?

Choose the letter A, B, C or D the word that has the underlined part different from others.

A. do

B. go

C. to

D. move

Cường độ âm tại một điểm trong môi trường truyền âm là  10 − 5   W / m 2 . Biết cường độ âm chuẩn là  I 0 = 10 − 12   W / m 2 . Mức cường độ âm tại điểm đó là

A. 70 dB

B. 80 dB

C. 60 dB

D. 50 dB

Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Because writing has become so important in our culture, we sometimes think of it as more real than speech. A little thought, however, will show why speech is primary and writing secondary to language. Human beings have been writing at least 5,000 years, but they have been talking for much longer, doubtless ever since there have been human beings. When writing developed, it was derived from and represented speech, although imperfectly. Even today, there are spoken languages that have no written form. Furthermore, we all learn to talk well before we learn to write; any child who is not severely handicapped physically or mentally will learn to talk: a normal man cannot be prevented from doing so. On the other hand, it takes a special effort to learn to write; in the past, many intelligent and useful members of society did not acquire the skill, and even today many who speak languages with writing systems never learn to read or write while some who learn the rudiments of those skills do so imperfectly.

To affirm the primacy of speech over writing is not to disparage the later. One advantage writing has over speech is that it is more permanent and makes possible the records that any civilization must have. Thus, if speaking makes us human, writing makes us civilized.

According to the passage, writing _______ .

A. is imperfect, but less than speech

B. represents speech, but not perfectly

C. developed from imperfect speech

D. is represented perfectly by speech