Different densities have to have a reason - different pressure and/or humidity etc. If there is a different pressure, there is a mechanical force that preserves the pressure difference: think about the cyclones that have a lower pressure in the center. The cyclones rotate in the right direction and the cyclone may be preserved by the Coriolis force.
If the two air masses differ by humidity, the mixing will almost always lead to precipitation - which includes a phase transition for water etc. It's because the vapor from the more humid air mass gets condensed under the conditions of the other. You get some rain. In general, intense precipitation, thunderstorms, and other visible isolated weather events are linked to weather fronts.
At any rate, a mixing of two air masses is a nontrivial, violent process in general. That's why the boundary is called a "front". In the military jargon, a front is the contested frontier of a conflict. So your idea that the air masses could mix quickly and peacefully - whatever you exactly mean quantitatively - either neglects the inertia of the air, a relatively low diffusion coefficient, a low thermal conductivity, and/or high latent heat of water vapor. A front is something that didn't disappear within minutes so pretty much tautologically, there must be forces that make such a quick disappearance impossible.
C) a solid lower part and a liquid upper part
Explanation:
The physical nature of the earth's core is made up of a solid lower part and a liquid upper part.
The core is the innermost part of the earth and it is made up of metallic minerals.
- It has the highest temperature and pressure of all the layers of the earth.
- The core is divided into two. Outer and inner core.
- Outer core is made up of molten metallic minerals. It is the layer where the earth geomagnetic field originates.
- The inner core is solid metallic ball.
- Evidence from seismic waves has furnished geoscientists with this knowledge.
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Your list of choices is a very short list, and doesn't include any
correct explanation.
The mass of an atom is roughly the number of protons AND neutrons
in its nucleus, but the element only depends on how many protons are there.
Different atoms of the same element may have different numbers of
neutrons, so their masses are different. But they're the same element,
because they all have the same number of protons.
Answer: the particles are more orderly in region 1
Explanation: region 1 is when the substance is a solid and as it is heated the particles move further apart and have more kinetic energy.