Question: \section{Passage $\mathrm{V}$}
Engineers and computer scientists are intrigued by the potential power of nanocomputing. Nanocomputers will use atoms and molecules to perform their functions instead of the standard integrated circuits now employed. Theorists believe that the amount of information a nanocomputer could handle is staggering.
A professor at Massachusetts Institute of Technology (MIT) has attempted to calculate the computational limits of a computer with a weight of 1 kilogram and a volume of 1 liter. According to the laws of physics, the potential amount of computational power is a function of the available energy. Basically, each atom and subatomic particle in the computer has an amount of energy attached to it. Furthermore, the energy of each particle or atom is increased by the frequency of its movement. Thus the power of a computer that uses nanotechnology is bounded by the energy available from its atoms.
Specifically, the relationship between the energy of an object and its computation potential is a proportionate one. As Einstein has famously calculated, the energy of an object is equal to its mass times the speed of light squared. Thus, a theoretical computer weighing a mere kilogram has a huge amount of potential energy. To find the total computational power, the minimum amount of energy required to perform an operation is divided by the total amount of energy. 
of energy possessed by a 1-kilogram computer by Planck's constant yields a tremendously large number, roughly $5 \times 10^{50}$ operations per second. Using even the most conservative estimates of the computing power of the human brain, such a computer would have the computational power of five trillion trillion human civilizations. The computer would also have a memory capacity, calculated by determining the degrees of freedom allowed by the state of all the particles comprising it, of $10^{31} \mathrm{bits}$.
These numbers are purely theoretical, however. Were the computer to convert all of its mass to energy, it would be the equivalent of a thermonuclear explosion. And it is unreasonable to expect human technology to ever achieve abilities even close to these limits. However, a project at the University of Oklahoma has succeeded in storing 50 bits of data per atom, albeit on only a limited number of atoms. Given that there are $10^{25}$ atoms in 1 kilogram of material, it may be possible to fit up to $10^{27}$ bits of information in the computer. And if scientists are able to exploit the many properties of atoms to store information, including the position, spin, and quantum state of all its particles, it may be possible to exceed even that number.
One interesting consequence of such staggering increases in computing power is that each advance could provide the basis for further evolution. Once technology can achieve, for instance, a level of computation equal to $10^{40}$ operations per second, it can use that massive power to help bring the theoretical limit ever closer.
The central thesis of the passage is:

Options:
A. Computing power is limited only by the laws of physics.
B. New advances in computer technology allow for staggering levels of memory and computational ability.
C. It may not be possible to achieve the theoretical limits of computing power.
D. Computers using nanotechnology have the potential to tap vast quantities of power.
Answer: This question asks you to find the main idea. This passage Passage $\mathrm{V}$ explains that theoretically, a massive amount of computational power is available. However, practical considerations make it unlikely that this limit would ever be reached. Thus choice $\mathrm{C}$ is the best answer. Choice $\mathrm{A}$ is a detail provided in the passage, but it is not the main idea. Choice $\mathrm{B}$ is incorrect, because the advances discussed are still primarily theoretical. Choice $\mathrm{D}$ is also only partly correct, because it doesn't deal with the practical limits mentioned in the passage. The correct answer is C.