我的问题是如何计算pandas中多个变量的频率。我从这个数据框中得到:d1=pd.DataFrame({'StudentID':["x1","x10","x2","x3","x4","x5","x6","x7","x8","x9"],'StudentGender':['F','M','F','M','F','M','F','M','M','M'],'ExamenYear':['2007','2007','2007','2008','2008','2008','2008','2009','2009','2009'],'Exam':['algebra','stats','bio','alg