许多学生在考试前会喝含咖啡因的饮料，因为它是一种有效的提神剂。

Many students drink caffeinated beverages before exams as they contain a strong ___stimulant to help them stay alert.

新推出的教育政策对学生的学习热情起到了激励作用。

The new education policy served as a ___stimulant for students' learning enthusiasm.