Sentence:On 23 April 1945, the hunter-killer group, operating as a combined force against a large [e1]wolfpack[/e1] of [e2]U-boats[/e2], spotted a partially submerged submarine but could not locate it after it dived.
 e1: wolfpack. 
 e2 : U-boats.
Answer: Member-Collection(e2,e1)