头孢菌素具有以下特点

A．抗菌谱广

B．杀菌力强

C．对β-内酰胺酶稳定

D．耐青霉素酶

E．过敏反应少

An economist is developing a regression model of real GDP as a function of labor input. She needs to select a time series that represents the total amount of labor performed in a year. Which of the following is the most appropriate indicator of total labor performed that she can use for this model（）

A. Aggregate hours.

B. Average workweek.

C. Number of employed.