The name of that mineral-rich silt is loess. In fact, this extensive Chinese river is called Huang ("Yellow") due to the great amounts of fine loess deposits that it carries to the Bohai Sea and that turn the water that color. It is not infrequent that, as a result of these thick loess sediments, the course of the river changes, and that the bed of the river is so elevated as to cause devastating, and often deadly, floods.
A "stimulus" is basically an event that happens in the outside environment that triggers a response. So, when you touched the dog's ear, that's a stimulus. The twitch is a response! So, A is correct. But just to be sure, let's look at the rest.
Artificial selection (B) is basically breeding: when humans breed organisms in order to obtain desired traits, like breeding dog species--this is not what our example is talking about.
Natural selection (C) is a similar process. When an organism mutates, but it actually helps it survive, it is more likely than the others to survive long enough to have babies. These babies are likely to have the helpful mutation, and so are their babies, and theirs, until a good portion of them have the mutation. This is NOT the example above!
And D, reproduction--well, let's just say it would involve a female dog and lots of babies, but that's not related to our example, so that isn't right either. The answer is A: a response to a stimulus.
Answer: A
In a FEDERAL REPUBLIC citizens elect the president as head of government separately from the legislature. In this kind of political organization, the state is divided in three powers, legislative, executive and judicial. The first one is lead by the president elected by the people as well as the senators and deputies who are part of the legislature. This division of powers allows the control between each other without abusing of their own power.
