Ais scientific knowledge established through direct observation and remains constant. scientific knowledge can change when scientists

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This is where the blanks are: A is scientific knowledge established through direct observation and remains constant. Scientific knowledge can change when scientists

The answer is followiing.

A law is scientific knowledge established through direct observation and remains constant. Scientific knowledge can change when scientists find new evidence.

Explanation:

A scientific law describes the relation (usually in mathematical terms) of natural phenomena and is supported by numerous observations and experiments. The laws are established when all the observations and result show that all the facts obey the same relation. For instance, the Universal law of gravitation: all the objects are attracted with a force that is directly proportional to the product of their masses and inversely related to the square of the distance that separates them: in lay terms, everything that goes up has to go down, and you do not expect that when a parachutist jumps out of an airplane he or she does not fall on the ground.

On the other hand, a scientific theory, although is an explanation that has been repeatedly tested and verified, is always under scientific scrtutiny and, as additional evidence is gathered it can be modified or replaced by a new theory. That is why it is said that scientific theories have to be testable and falsiable: if it is not testable and falsiable then it  is not a scientific theory.

Scientific theories are aimed to describe, explain and make predictions about the objectivable world.

tracks, burrows, eggshells, nests, tooth marks, or gastroliths

the answer is na (sodium)