A greenhouse gas is a gas in an atmosphere that absorbs and emits radiant energy within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. 

Carbon dioxide is a main greenhouse gas, found in car exhaust pipes.
Methane and nitrous oxide is another type of greenhouse gas.

Oxygen is the only clean gas out there because we need it to survive. Therefore, the only gas that is not a greenhouse gas is Oxygen.

An intensive property is a physical quantity whose value does not depend on the amount of the substance for which it is measured. For example, the temperature of a system in thermal equilibrium is the same as the temperature of any part of it. If the system is divided the temperature of each subsystem is identical. The same applies to the density of a homogeneous system, if the system is divided in half, the mass and the volume change in the identical ratio and the density remains unchanged. Additionally, the boiling point of a substance is another example of an intensive property. For example, the boiling point for water is 100 °C at a pressure of one atmosphere, a fact which remains true regardless of quantity.

What has 4 Eyes But cannot see is Mississippi.

The wording of Mississippi has 4 'i''s. But it can't see.

C++ is not an Operating System.

An Operating System is a collection of system programs that manages all the other programs application programs in a computer as well as the allocation and use of hardware resources such as the CPU, Memory and the Hard Disk Drive.

It acts as an interface between the hardware and the user level program.

It controls and facilitates the overall operation of a computer. 

Here

UNIX : UNIX is a popular multi-user, multitasking operating system (OS) developed at Bell Labs.

Windows NT : Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993.

DOS : DOS is Disk Operating System. The term DOS can refer to any operating system.

C++ : C++ is a programming language and computing platform.

Hence, C++ is not an Operating System.

Answer for What City has No People can be any of these words which has city in it like Velocity, Elasticity or Electricity.

In general the answer for the riddle What City has No People is Electricity.

What has one eye yet cannot see is An Eye of a Needle.

A needle has an eye through which we thread it but it is not for seeing.

What has Hands But cannot Clap is a Clock or a Watch.

A Clock technically has 3 hands.

1 Hour Hand Which tells Hours

1 Minute Hand Which tells Minutes

1 Second Hand Which tells Seconds.

But cannot clap with these hands.

Monoculture is a typical characteristics of Commercial grain farming.

Monoculture is the agricultural practice of producing or growing a single crop, plant, or livestock species, variety, or breed in a field or farming system at a time focussing commercial gains.

Monoculture is widely used in both industrial farming and organic farming and has allowed increased efficiency in planting and harvest.

Polyculture, where more than one crop is grown in the same space at the same time, is the alternative to monoculture.