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

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

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" } } }] )

A.Key

B.Code block

C.None of the above

D.Name/value pair

A.$push along with $each, $sort and $slice

B.$arrayLimit

C.None of the above

D.$removeFromSet

A.No limit on document and 100 MB RAM

B.16 MB document and 100 MB RAM

C.2 MB document and 100 MB RAM

D.2 MB document and no limit on RAM