A.db.test.ensurelndex( { c:l, a: 1})

B.CH db.test.ensurelndex( { a :1, c:l})

C.db.test.ensurelndex( { a: 1, b :1, c:l, d:l})

D.db.test.ensurelndex( { a : 1, c: 1, d: 1, b : 1})

A.$ elemMatch

B.Updating an array field without knowing its index is not possible.

C.$slice

D.$

A.All of the above

B.Map-reduce

C.Issuing a query

D.Inserting data

A.$modify

B.$set

C.$project

D.$update

A.db.population.aggregate( [{ $group: { _id: { state: Estate", city: "$city" },pop: { $sum: 1 > > >] )

B.db.population.aggregate( [{ $group: { _id: { city: "$city" },pop: { $sum: "$pop" } } }] )Multi Document Transaction is not supported by MongoDB

C.db.population.aggregate( [{ $group: { _id: { state: "$state", city: "$city" },pop: { $pop: 1 } } }] )

D.db.population.aggregate( [{ $group: { _id: { state: Estate", city: "$city" },pop: { $sum: "$pop" } } }] )