She developed paralysis(瘫痪), after giving birth to a large calf. That changed her status from milk producer to "downer", the dairy industry's word for a cow unable to stand up. The United States Department of Agriculture (USDA) estimates that downers account for between 130,000 and 195,000—a large percentage—of the total cows slaughtered each year.
Her owner, the Sunny Dene Ranch dairy farm, sent her to Veru's Moses Lake Meats. She was given a routine test for nervous-system diseases, killed, then shipped to Midway Meats in Centralia. The test, taken on December 9th but not fully analyzed until December 22nd, froze America's $27-billion-a-year cattle industry, the world's largest producer of beef. This cow had tested positive for BSE(牛绵状脑病),a fatal sickness popularly known as mad cow disease. Humans can get a different kind of disease of BSE by consuming the brains or nervous-system tissue of infected cattle. Britain diagnosed its first case of BSE in 1986. It eventually spread to 180,000 cattle and was linked to the deaths of nearly 150 people.
The USDA at once quarantined(隔离) the Sunny Dene herd and made efforts to recall the meat. Two dozen countries equally swiftly slammed the door on American beef experts, which earn $3 billion a year. The Asian market, particularly Japan, which buys about a third of those exports, was especially quick to ban American meat. Cattlemen cursed their bad luck after a year of decent prices.
On December 27th the USDA suggested that the infected cow came from Canada. This does not let the American beef industry off the hook. But it suggests that the cause of the trouble lies outside the United States. It was not such good news for Canada. Last May one case of BSE in northern Canada led to a three-month ban on exports of Canadian beef and live cattle. When will the cattlemen get their market back.'？ The Americans hastily sent a team to Japan to try to calm Japanese fears and, more practically, to discuss the import of beef from animals less than 30 months old. Steve Kay, an expert on the cattle industry from Petaluma, California, points out that BSE has never been found in cattle younger than 30 months.
Much as officials once did in Britain, people from the USDA and the meat industry have rushed to assure Americans that their product is safe. Shares in McDonald's at first tanked, but recovered when the burger giant insisted it used strict safety standards. Yet America's meat industry still has to pay more attention to what it feeds its animals and how it kills them.
What cows constitutes high percentage in the total cows butchered each year？

A. Cows that have give birth to calves.
B. Cows that have produced milk.
Cows that are less than 30 months old.
D. Cows that cannot stand up.