如果研究的因果关系只涉及一个因变量和一个自变量，这种回归分析法称为（）。A、一元回归

     Counterfeit (假的) medicines are a widespread problem in developing countries. Like other counterfeits,
they look like real products. But counterfeit drugs may contain too little or none of the active ingredients
(原料) of the real thing.
     People do not get the medicine they need. And in some cases the counterfeits cause death. Twenty
children in Bangladesh died last year after being given acetaminophen (醋氨粉). The medications contained
ingredients that looked, smelled and tasted like the real thing. The medicine was produced by a local drug
company that used a dangerous substitute to save money.
     The problem with counterfeit medicines is especially bad in Africa, Asia and Latin America. The
W. H. O. estimates that up to thirty percent of the medicines on sale in many of those countries are
counterfeit.              .
     The W. H. O. says counterfeits make up less than one percent of the illegal drug market in countries like
the United States, Canada, Japan, and New Zealand.
     But the agency also says as much as fifty percent of the medicine sold on the Internet is counterfeit.
     Much is being done to fight counterfeit drugs. Several companies are developing ways to make
counterfeits easier to identify. And there are existing methods, like a machine that can quickly identify
chemicals in pills to confirm if the pills are real. Other ideas include things like special tracking codes for drug
packages. People could send a text message with the code and get a message back proving that what they
bought is listed in a database. Some drug makers and other companies put three-dimensional images called holograms (条形码) on their products as a security device.