Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Continents and ocean basins represent the largest identifiable bodies on Earth. On the solid portions of the planet, the second most prominent features are flat plains, elavated plateaus, and large moutain ranges. In geography, the term "continent" refers to the surface of continuous landmasses that together comprise about 29.2% of the planet's surface. On the other hand, another definition is prevalent in the general use of the term that deals with extensive mainlands, such as Europe or Asia, that actually represent one very large landmass. Although all continents are bounded by the water bodies or high mountain ranges, isolated mainlands, such as Greenland and India-Pakistan areas are called subcontinents. In some circles, the distinction between continents and large islands lies almost exclusively in the size of a particular landmass.

The analysis of compression and tension in the earth's crust has determined that continental structures are composed of layers that underlie continental shelves. A great deal of disagreement among geologists surrounds the issue of exactly how many layers underlie each landmass because of their distintive mineral and chemical composition. It's also quite possible that the ocean floor rests on top of unknown continents that have not yet been explored. The continental crust is believed to have been forrmed by means of a chemical reaction when lighter materials separated from heavier ones, thus settling at various levels within the crust. Assisted by the measurements of the specifics within crust formations by means of monitoring earthquakes, geologists can speculate that a chemical split occured to form the atmosphere, sea water and the crust before it solidified many centuries ago.

Although each continent has its special features, all consist of various combinations of components that include shields, moutain belts, intracratonic basins, margins, volcanic plateaus, and blockvaulted belts. The basic differences among continents lie in the proportion and the composition of these features relative to the continent size. Climatic zones have a crucial effect on the weathering and formation of the surface features, soil erosion, soil deposition, land formation, vegetation, and human activities.

Mountain belts are elongated narrow zones that have a characteristic folded sedimentary organization of layers. They are typically produced during substabtial crustal movements, which generate faulting and moutain building. When continental margins collide, the rise of a marginal edge leads to the formation of large moutain ranges, as explained by the plate tectonic theory. This process also accounts for the occurrence of mountain belts in ocean basins and produces evidence for the ongoing continental plate evolution.

According to tha passage, how do scientists define continents?