What is the most common form of non鄄tariff barriers? Explain it

What is the most common form of non鄄tariff barriers? Explain it in a few words.
【正确答案】：Quotas or quantitative restrictions are the most common form of non-tariff barriers. A quota limits the imports or exports of commodity during given period of time.The limits may be in quantity or value terms and quotas may be on country basis or global without reference to countries.