The latest clock to be invented is a "language clock" which helps us to determine the date

In 1950 some research workers in America discovered that languages change at a steady rate through the centuries. They organized their investigation in the following ways. First, they prepared a list of 200 things for which there are common words in every language. Then they compared these words in modern languages with the ancient languages from which the modem ones had developed. They found that for a certain proportion of the 200 things, the ancient words continued to be used, though written or pronounced differently. But in other cases the ancient words had been replaced by new words which had been introduced into the language.

After a thousand years, on the average, 81% of the old words (162 of the 200 words) were still in use. After 2000 years, on the average, 81% of these 162 words (or 66% of the original to tal of 200) remained. After 3000 years, 81% of these 131 words (or 53% of the original number) remained in use, and so on.

Next this group of research workers investigated situations where two different languages had developed separately and independently, from the same ancient language, (In modern times, of course, such situations are very unlikely to occur. Why？) They found that after 1,000 years, as be fore, each daughter language shared only 66% of the basic 200-word vocabulary. Each daughter language, as before, derived 81% of its 200 words from the original parent language. But (as you might expect) the words which they kept were not quite the same. The proportion of words actually shared by the daughter languages was therefore 81% of 81%, which is 66% of the original 200 words. After 2000 years they shared 66% of 66% of the words, i.e. 43% of the original vocabulary. And after 3000 years they shared only 29% of the original vocabulary.

A long time ago, some Eskimos, speaking the Eskimo language, left the mainland of Alaska and began to live in a group of islands (called the Aleutian Islands) at some instance from the coast. Recently, research workers compared the islander's language with the modern Eskimo language. They found that the Aleutians and the Eskimos shared only 29% of the words, on the stand ard list. From this fact the investigators were able to calculate the date of the event referred to at the beginning of this paragraph.

From the passage, we can infer that language clock is ______.

A．a mechanic clock

B．a figurative clock

C．an electronic clock

D．a diamond clock