Passage One
Questions 21 to 25 are based on the following passage.
During the early years of this century，wheat was seen as the very lifeblood of Western Canada. When the crops were good，the economy was good；when the crops failed. there was depression. People on city streets watched the yields and the price of wheat with almost as much feeling as if they were growers. The marketing of wheat became an increasingly favorite topic of conversation.
War set the stage for the most dramatic events in marketing the western crop. For years farmers mistrusted speculative(投机的)grain selling as carried on through the Winnipeg Grain Exchange. Wheat prices were generally low in the autumn，but farmers could not wait for markets to improve. It had happened too often that they sold their wheat soon after harvest when farm debts were coming due，only to see prices rising and speculators getting rich. On various occasions，producer groups asked for firmer controls，but governments had no wish to become involved，at least not until wartime wheat prices threatened to run wild.
Anxious to check inflation(通货膨胀)and rising living costs，the federal government appointed a board of grain supervisors(监视员)to handle deliveries from the crops of 1917 and 1918. Grain Exchange trading was suspended，and farmers sold at prices fixed by the board. To handle the crop of 1919，the government appointed the first Canadian Wheat Board，with full authority to buy，sell. and set prices.
The author uses the term “lifeblood” to indicate that wheat was______.
A. difficult to produce in large quantities
B. susceptible to many parasites(寄生虫)
C. essential to the health of the country
D. expensive to gather and transport