我需要按组替换仅一列（例如x2）的缺失观测值。
set.seed(123)
df <- data.frame(matrix(rnorm(100), ncol = 5))
df$Group <- letters[1:20]
df <- df[rep(seq_len(nrow(df)), sample(1:10, 20, replace = T)),]
df_complete <- df
df$X1[sample(1:nrow(df), 15)] <- NA
df$X2[sample(1:nrow(df), 10)] <- NA
df$X3[sample(1:nrow(df), 25)] <- NA
df$X4[sample(1:nrow(df), 10)] <- NA
df$X5[sample(1:nrow(df), 15)] <- NA