Đề luyện đọc Tiếng Anh trình độ B1 - Đề số 14

On a drop shot, a tennis player “drops” the ball just over the net, hoping that his or her opponent won’t get to it at all or will just barely reach it, thus making a weak return. The drop shot works well in a number of situations. It can be used to tire an opponent, to bring a baseline player to the net, to win points outright when an opponent is slow in moving forward or is out of position, or to substitute for the approach shot.

A perfect situation for a drop shot occurs when a player’s opponent is far out of court and hits well to the inside of the service line. A good drop shot is a sure winner, but a bad one is equally certain disaster. The opponent who gets to the ball early has been handed the net position, which is a distinct advantage for the net rusher who will usually win the point in short order.

There are two types of drop shots, each requiring a distinct stroke. The first is used to drop slow balls descending from the peak of the bounce. The second is used on rising balls. These shots require excellent timing and a simple stroke, such as the swing on waist-high volleys.

A snowflake originates from countless water molecules that initially come together in small groups as a result of a weak attractive force between oxygen and hydrogen atoms. The same forces subsequently organize the groups into a frozen molecular crystal, a perfectly organized lattice of molecules. Finally, several molecular crystals join to form a snowflake. Scientists have realized for some time that the forces that assemble molecules into natural crystals can be utilized to produce a variety of important materials. They have determined the structure of more than 90,000 different molecular crystals, the most common examples of which are aspirin and mothballs.​

In recent years, researchers have studied how molecules organize themselves to form crystals in the hope of better understanding what types of molecules and what conditions will produce molecular crystals with unusual and useful properties. Scientists are aware that the material properties of a crystal depend in large part on the organization of the molecules in the crystal, yet they know little about the factors controlling the assembly of such crystals.​

Synthesizing a molecular crystal is similar to designing a building. Before construction can begin, the architect must specify the shapes and sizes of the girders and the number and placement of the rivets. Similarly, to produce new molecular crystals, chemists must choose molecules of the appropriate sizes and shapes and select the molecular forces that will hold the crystals together. A chemist can normally find many molecules of various shapes and sizes, but the challenge is to find ones that assemble in a predictable manner.​