The United States Department of Labor (enforces) laws that (promote) the welfare of wage earners, (improving) occupational conditions and (advance) employment opportunities.

单项选择题According to the passage, the experience of helplessness causes rats to ______

单项选择题Part Ⅳ ClozeDirections: In this part, there is a passage with 15 blanks. For each blank, there are 4 choices marked A, B, C and D. Choose the best answer for each blank and mark the corresponding letter with a single bar across the square brackets on your machine-scoring ANSWER SHEET.