Answer:
It's held together by the nuclear force.
Explanation:
There are <em>more</em> elemental forces than just the electromagnetic one. In this case, it is the nuclear force (called also strong force) the one that holds the nucleus together because it is stronger than the electromagnetic force over such short distances as the one inside the atomic nucleus.
Newton's second law states that the resultant of the forces applied to an object is equal to the product between the object's mass and its acceleration:

where in our problem, m is the mass the (child+cart) and a is the acceleration of the system.
We are only concerned about what it happens on the horizontal axis, so there are two forces acting on the cart+child system: the force F of the man pushing it, and the frictional force

acting in the opposite direction. So Newton's second law can be rewritten as

or

since the frictional force is 15 N and we want to achieve an acceleration of

, we can substitute these values to find what is the force the man needs:
I believe the answer is C
The answer is invasive species. Invasive species is when plants, fungus or animals that are not familiar or native to specific location and has a tendency to spread damage to environment, human health and human economy. eg: Cane toads are originally from Australia and was moved to grasslands. These toads are invasive species in Grasslands because they are destroying the environment and destroying the balance of wildlife (in Grasslands).
Invasive species are one of the threats to biodiversity and some ecosystems.
hope this helps
Rusting is a chemical change, which forms a new substance which is brittle. A solid that is formed by a chemical reaction.