People thinking about the origin of language for the first time usually arrive at the conclusion that it developed gradually as a system of grunts, hisses and cries and【C1】______ a very simple affair in the beginning.【C2】______ ,when we observe the language behaviour of【C3】______ we regard as primitive cultures, we find it【C4】______ complicated. It was believed that an Eskimo must have at the tip of his tongue a vocabulary of more than 10 000 words【C5】______ to get along reasonably well, much larger than rite active vocabulary of an average businessman who speaks English.【C6】______ ,these Eskimo words are 【C7】______ more highly inflected than【C8】______ of any of the well-known European languages, for a【C9】______ noun can be spoken or written in【C10】______ hundred different forms, each【C11】______ a precise meaning different from that of any other. The【C12】______ of the verbs are ever more【C13】______ .The Eskimo language is, therefore, one of the most difficult in the world to learn,【C14】______ the result that almost no traders or explorers have【C15】______ tried to learn it.【C16】______ ,there【C17】______ grown up, in communication between Eskimos and whites, a jargon.【C18】______ to the pidgin English used in Old China, with a vocabulary of from 300 to 600 uninflected words. Most of them are derived from Eskimo【C19】______ some are derived from English, Danish, Spanish, Hawaiian and other languages. It is this jargon that is usually 【C20】______ by travelers as "the Eskimo language".
【C1】

A. must be
B. must have been
C. ought to be
D. should be