<u>Answer:</u>
<em>Anytime observations need to be clarified. When an unusual condition need to be shown for later reference.</em>
<u>Explanation:</u>
Instead of just recording the data the is sometimes important to be sketched out as the data observed is needed to be clarified and also sometimes there is an unusual condition that occurs and hence, it is important to sketch it so that it can be used for later reference.
<em>Therefore, when we are aware of the situation we need not sketch the data.</em>
Answer:Geographic featured that acted as norther borders of the Roman Empire at its height were: Rhine River, Danube River, Carpathian Mountains, and the Black Sea.
Erosion can be controlled by building terraces. The correct option among all the options that are given in the question is the first option. The building of terraces prevents soil erosion by a good margin. It is mostly done in the hilly regions to stop erosion of soil by rain. I hope the answer helps you.
<span>A greenhouse gas is a gas that contributes to the greenhouse effect by absorbing infrared radiation. It is important because it keeps the Earth's surface heated and able to maintain life upon it.</span>
