将数据从三列reshape 为两列

我有一个这样的数据:

   Response Prompt Prompt.Label Comments
1    Friend    who          si3      3_a
2         1    was       si4_en      3_a
3         1    was      si4_nag      3_a
4    Family    who          si3      3_b
5         1    was      si4_nag      3_b
6         1    was       si4_en      3_b
7     other    who          si3      4_a
8         1    was       si4_sp      4_a
9     other    who          si3      4_b
10        1    was       si4_en      4_b
11   Friend    who          si3      5_a
12        1    was       si4_en      5_a
13   Family    who          si3      5_b
14        1    was       si4_en      5_b

data<-structure(list(Response = c("Friend", "1", "1", "Family", "1", 
"1", "other", "1", "other", "1", "Friend", "1", "Family", "1"
), Prompt = c("who", "was", "was", "who", "was", "was", "who", 
"was", "who", "was", "who", "was", "who", "was"), Prompt.Label = c("si3", 
"si4_en", "si4_nag", "si3", "si4_nag", "si4_en", "si3", "si4_sp", 
"si3", "si4_en", "si3", "si4_en", "si3", "si4_en"), Comments = c("3_a", 
"3_a", "3_a", "3_b", "3_b", "3_b", "4_a", "4_a", "4_b", "4_b", 
"5_a", "5_a", "5_b", "5_b")), class = "data.frame", row.names = c(NA, 
-14L))

我希望reshape 数据有两列，包括源和目标.当"Prompt"=="Who"时，"Source"的值应该来自"Response"；如果"Comments"列的每个值的"Prompt.Label"的值都是"Prompt.Label"，则"Target"应该是"Prompt.Label"的值.例如，对于第一行，Friend代表"源"，si4_en代表"目标".

所需数据应为:

  Source  Target
1 Friend  si4_en
2 Friend si4_nag
3 Family si4_nag
4 Family  si4_en
5  other  si4_sp
6  other  si4_en
7 Friend  si4_en
8 Family  si4_en