在MongoDB中，对于集合 A { "field1" : "val1", "field2" : "val2" }，如果field2是枚举，请选择优化更新集合的最佳方法。( )
{ “field1” : “val1”, field2 : [“val2”] }
{ field1 : [“val1”], field2: {“field3”:...} }
{ field1: {“field2”:...} , field3: {“field4”:...} }
{ “field1” : “val1”, “field2” : “val2” }
