猪肉的消费需求（Q），随着饲料价格（P）、家庭收入（T）、猪肉价格（E）、屠宰税（I）的变动而变化，下列选项能正确反映其变动关系的是

①                 ②                ③                  ④

A．①②③

B．①②④

C．①③④

D．②③④

Our foreign English teacher Shirley has a gift for creating an atmosphere for her students _________ allows them to communicate freely with each other.

A．which

B．where

C．what

D．who