df_c<- student %>%   group_by(sid, year) %>%   summarise(min_grade = min(grade), ave_min_grade = mean(min_grade)) df_d<- student %>%   group_by(sid, year) %>%   summarise(min_grade = min(grade)) %>%   summarise(ave_min_grade = mean(min_grade))   Number of rows in df_c is different from that in df_d. There are 4 columns in df_c while there are 2 columns in df_d.  多重下拉选择题

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