单选题 According to news reports, US restaurants are partly to blame for the growing bellies. Awaiter puts a plate of food in front of each customer, with two to four times the amount recommended by the government, according to a USA Today story. Americans traditionally associate quantity with value and most restaurants try to give them that. They prefer to have customers complain about too much food rather than too little.
Barbara Roils, a nutrition professor at Pennsylvania State University, told USA Today that restaurant portion sizes began to grow in the 1970s, the same time that the American waistline began to expand.
The average waistline of Americans started to grow

• A. two or three years ago.
• B. in the 1970s.
• C. about four years ago.
• D. since last year.