Questions 17-20 are based on the following passage.

    Spider-Man isn’t the only person with an interest in spider silk. While Spider-Man uses the threads to zigzag from building to building or to snare a bad guy, scientists are investigating silk for different reasons. Though researchers have learned a lot about silk by investigating spiders, insects such as caterpillars, ants, and bees also have been studied for the sticky stuff. Scientists are even trying to get silk from animals such as goats.

    It turns out silk might be good for weaving a lot more than shirts and ties. In the future, the silky fiber might be used to make super tough bulletproof vests and light but strong parachute cords. Silk also might work well for delicate tasks inside the body. Researchers are experimenting with using silk to support growing cells, the same way a construction crew builds scaffolding around a building to help keep everything in its place during construction.

    Spider silk is an ideal material, according to a researcher from University of Wyoming, but humans have been gathering silk not from spiders but from silkworms for hundreds of years. But silkworm silk has its problems. A silkworm covers its silk in sticky glue that holds the cocoon together. Sometimes humans have a bad allergic reaction to this glue.

    Spiders, on the other hand, don’t use sticky glue. Most spiders have an abdomen made up of five different sections. The last two sections are where the silk-making happens. These sections of the lower belly are modified into special structures called spinnerets, which are sort of like faucets for silk. The silk is mixed in glands and then secreted out of the spinnerets. Spiders can’t shoot silk out for long distances the way Spider-Man does. Instead, they attach the emerging silk to something, like a tree branch, and then move away from the branch. This pulls the silk outward.

    The main ingredient in spider silk is proteins, and there are many different kinds, depending on which spider is spinning and which silk it wants to make. Some of the proteins are very large and complicated, and therefore hard to make a lot of in the lab. So some scientists have put the genes that have the instructions for making silk into other creatures, such as goats. The silk-making genes are turned on only in the goat cells that make milk, so when these goats are milked, there is silk in the milk. However, a liter of milk may have only 15 grams of silk, which means it would take about 600 gallons of milk to make one bulletproof vest. At higher concentrations, the milk starts clumping, perhaps because the silk proteins are sticking to milk proteins.

The word clumping in paragraph 5 is best replaced by ....