In this task, you are given a question and a context passage. You have to answer the question based on the given passage.

What type of equipment is used with liquid oxygen but not other methods of oxygen storage?, Context: Oxygen storage methods include high pressure oxygen tanks, cryogenics and chemical compounds. For reasons of economy, oxygen is often transported in bulk as a liquid in specially insulated tankers, since one liter of liquefied oxygen is equivalent to 840 liters of gaseous oxygen at atmospheric pressure and 20 °C (68 °F). Such tankers are used to refill bulk liquid oxygen storage containers, which stand outside hospitals and other institutions with a need for large volumes of pure oxygen gas. Liquid oxygen is passed through heat exchangers, which convert the cryogenic liquid into gas before it enters the building. Oxygen is also stored and shipped in smaller cylinders containing the compressed gas; a form that is useful in certain portable medical applications and oxy-fuel welding and cutting.
heat exchangers

How does a chestnut tree form its heartwood?, Context: The term heartwood derives solely from its position and not from any vital importance to the tree. This is evidenced by the fact that a tree can thrive with its heart completely decayed. Some species begin to form heartwood very early in life, so having only a thin layer of live sapwood, while in others the change comes slowly. Thin sapwood is characteristic of such species as chestnut, black locust, mulberry, osage-orange, and sassafras, while in maple, ash, hickory, hackberry, beech, and pine, thick sapwood is the rule. Others never form heartwood.
the change comes slowly

PNB has a what for education?, Context: Seattle has been a regional center for the performing arts for many years. The century-old Seattle Symphony Orchestra is among the world's most recorded and performs primarily at Benaroya Hall. The Seattle Opera and Pacific Northwest Ballet, which perform at McCaw Hall (opened 2003 on the site of the former Seattle Opera House at Seattle Center), are comparably distinguished, with the Opera being particularly known for its performances of the works of Richard Wagner and the PNB School (founded in 1974) ranking as one of the top three ballet training institutions in the United States. The Seattle Youth Symphony Orchestras (SYSO) is the largest symphonic youth organization in the United States. The city also boasts lauded summer and winter chamber music festivals organized by the Seattle Chamber Music Society.

School