PSAT Writing and Language Practice Question 704

Question: 704

Glassmaking
Have 12 you ever noticed something strange about the glass in the windows of old buildings? Look closely, and you'll see that their surfaces, though smooth, have easily discernible inconsistencies in width, especially near the edges. As you look through the glass, you'll see that, like a correctional lens, these imperfections distort the images that pass through them. 13 Well into the 19th century, glass windows were often made using a technique called "glassblowing." Glassblowing is a very ancient craft, the earliest known instances 14 in which date back to more than 5,000 years ago in Egypt and Eastern Mesopotamia. 
[1] Though a skilled glassblower can create remarkable (and almost perfectly symmetrical) items like vases, cups, and globes, the process of making a flat pane of glass can be particularly tricky. [2] 15 Traditionally, a bowl-shaped glob of molten glass was flattened into a disk using centrifugal force and then cut to the proper shape and size. [3] However, the viscosity of liquid glass is such that, as spinning progresses, the perimeter of the disk becomes much thicker than the center. [4] In these glass factories, glass is poured slowly into the center of a steel table and allowed to spread under the influence of gravity. [5] Panes created using this technique are usually identifiable for their bulbous centers and wavy surface 16 texture, additionally they are often very fragile, as the edges joining with the window frame are typically narrower than the pane's body. 17 
Today we can produce even very large sheets of glass of nearly uniform thickness using the "float glass" process invented by Sir Alastair Pilkington in the mid-1950s. As the name implies, this technique 18 involve floated molten glass on a bath of molten tin. The atmospheric composition within a float furnace must be strictly regulated to prevent the tin from oxidizing. As glass enters the bath, its specific gravity and immiscibility with tin 19 causes it to form a continuous ribbon with perfectly smooth surfaces on both sides and an even width throughout. The glass is then gradually cooled until it can be lifted from the tin bath. Typically, its temperature at this time is around 1,100°C. The tin, meanwhile— 20 given its relatively high atomic number—remains in a fully molten state. The entire process is inexpensive, reliable, and 21 relatively cheap when compared to the laborious task of hand-spinning paned glass. However, a very close look can reveal imperfection even in this modern technique. If the glass is cooled 22 to quickly—as is sometimes the case in large-scale industrial production—the ribbon will absorb trace amounts of tin, leaving behind a faint haze on one side of the finished pane.

The writer wants to complete this description of the process with a third phrase that will contrast this process with what follows in the sentence. What choice best accomplishes the writer's goal?

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