For each student (ie. `sid`), find the average marks of the all courses taken. You are asked to have one average value for each student in the result. df2 <- student %>%   XXXX(sid) %>%   XXXX(ave_grade = mean(grade, na.rm = TRUE)) Please fill in the blanks below. df2 <- student %>%   [Fill in the blank] (sid) %>%   [Fill in the blank] (ave_grade = mean(grade, na.rm = TRUE)) 多项填空题

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