In chemistry, Weak bonds are those forces of attraction that, do not take a large amount of energy to break.

For example, hydrogen bonds are broken by energies in the order of 4 - 5 kcal/mol.

Ionic bonds, hydrogen bonds and van der Waals interactions are considered weak bonds.

Weak bonds are transient and easily reversible.

Bond breaking is an endothermic process.

Hence, bond breakage needs an amount of energy and uses some energy based on the strength of the bond to break the bond.

Here given the required digit number is 4 digit.

It must be divisible by 5. Hence, the unit's digit in the required 4 digit number must be 0 or 5. But here only 5 is available.

x x x 5

The remaining places can be filled by remaining digits as 5 x 4 x 3 ways.

Hence, number 4-digit numbers can be formed are 5 x 4 x 3 = 20 x 3 = 60.

Any number which can be expressed as a fraction of two integers like P & Q as P/Q where Q is not equal to zero.

Every integer is a rational number since Q can be 1.

Hence, in the given options, 4 can be expressed as a simple fraction as 4/1. And all other options cannot be expressed as fractions.

Hence, 4 is a rational number in the given options.

A computer can perform inputting, processing and cotrolling but it can't perform understanding as it is has no brain. It can't earn from its previous experiances.

Any factor limiting the size of a population whose effect is not dependent on the number of individuals in the population, then such factor is called Density Independent Factor. It is things that the organisms have no control over.

Here in the given options, food shortage and disease are factors that are dependent on the number of population but Floods have no dependency.

Some examples of density independent factors are :: Earth quake, fires, volcanic eruptions, floods, and  all natural calamities.

Gene flow means migration of genes from one to another. This gene flow is caused by individuals moving into population.

The Berlin Conference of 1884–85 was a meeting between European nations to create rules on how to peacefully divide Africa among them for colonization. The conference was convened by Portugal but led by Otto von Bismarck, chancellor of the newly united Germany. 

The 14 countries that attended included all the major European powers, as well as Russia, Turkey and the United States. The conference began with a dispute about the Congo River basin.Slavery was forbidden throughout Africa. The attendees agreed upon a principle of effective occupation by which rights over African colonies would be assessed.

Before the conference, about 80 percent of African territory was still in local hands. By 1902, European powers controlled 90 percent of Africa.