The cause for a molecular cloud forming fragments when it collapses is indicated correctly by option D. density variations from place to place grow larger as the cloud collapses.
Molecular cloud:
A molecular cloud, also known as a stellar nursery, is a specific kind of interstellar cloud, whose density and size allow the development of molecules, absorption nebulae, and H II regions. In contrast, some regions of the interstellar medium mostly consist of ionized gas.
Molecular clouds are cold, dense areas of space where stars form. The cloud collapses into a proto-star when the gravitational force pulling it in outweighs the internal pressure pushing it in.
When a molecular cloud collapses, it is observed that the density varies from place to place with the variation increasing with collapse. As a result, the collapse is characterized by fragmentation of the cloud.
Thus the correct option is: D. density variations from place to place grow larger as the cloud collapses.
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The work done is 375 J
Explanation:
The work done by a force in moving an object is given by

where
F is the magnitude of the force
d is the displacement
is the angle between the direction of the force and the displacement
In this problem,
F = 75 N is the force applied to the couch
d = 5 m is the displacement
Assuming the force applied to the couch is parallel to the motion, 
And so, the work done is

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Answer:
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Explanation:
An object that is partly, or completely, submerged experiences a greater pressure on its bottom surface than on its top surface. This causes a resultant force upwards. This force is called upthrust . The upthrust force is equal in size to the weight of the fluid displaced by the object.
Buoyancy or upthrust, is an upward force exerted by a fluid that opposes the weight of an immersed object.It is the force that pushes an object up. The upthrust, or buoyancy, keeps ships afloat. The upthrust, or buoyancy, keeps swimmers on top of the water.