Passage V
In soil, 
is produced through 2 processes—respiration in plant roots and bacterial decomposition of organic matter. A study was done in an oak forest to examine the content of soil gas as well as the water content of the soil. The study was done during an 8-week period that began just as the growing season ended.
Study
On October 12, 5 evenly spaced locations in the forest were marked along a 120 m long straight line, starting at one end. At each location, 5 sets of 2 instruments each—a diffusion well and a moisture sensor—were positioned so that soil gas could be collected and soil water content could be measured at each of 5 soil depths: 10 cm, 30 cm, 60 cm, 100 cm, and 140 cm. The slots near one end of the steel pipe of the diffusion well allowed only soil gas to enter the pipe. The soil gas could be sampled by inserting the needle of a syringe through the airtight seal on the aboveground end of the pipe. See Figure 1.
At noon on each of 4 dates—October 26, November 9, November 23, and December 7—a 0.5 mL sample of soil gas was collected from each diffusion well and the water content of the soil was read from each moisture sensor. Each soil gas sample was analyzed to determine its content. Figure 2 shows the averaged results for content of the soil gas, expressed in percent by volume, and Figure 3 shows the averaged results for water content of the soil, expressed in percent by mass.
29. The slots at the bottom of a diffusion well’s pipe were designed to allow the passage of:
Your Answer is
Correct Answer is A
Explanation
Look at the seventh line of the second paragraph of the article. The slots near one end of the steel pipe of the diffusion well allowed only soil gas to enter the pipe. So choose A