instruction:
In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
question:
Factories in China often produce what type of gas in their emissions that acts as pollution?, Context: A dense wave of smog began in the Central and Eastern part of China on 2 December 2013 across a distance of around 1,200 kilometres (750 mi), including Tianjin, Hebei, Shandong, Jiangsu, Anhui, Shanghai and Zhejiang. A lack of cold air flow, combined with slow-moving air masses carrying industrial emissions, collected airborne pollutants to form a thick layer of smog over the region. The heavy smog heavily polluted central and southern Jiangsu Province, especially in and around Nanjing, with its AQI pollution Index at "severely polluted" for five straight days and "heavily polluted" for nine. On 3 December 2013, levels of PM2.5 particulate matter average over 943 micrograms per cubic metre, falling to over 338 micrograms per cubic metre on 4 December 2013. Between 3:00 pm, 3 December and 2:00pm, 4 December local time, several expressways from Nanjing to other Jiangsu cities were closed, stranding dozens of passenger buses in Zhongyangmen bus station. From 5 to 6 December, Nanjing issued a red alert for air pollution and closed down all kindergarten through middle schools. Children's Hospital outpatient services increased by 33 percent; general incidence of bronchitis, pneumonia, upper respiratory tract infections significantly increased. The smog dissipated 12 December. Officials blamed the dense pollution on lack of wind, automobile exhaust emissions under low air pressure, and coal-powered district heating system in North China region. Prevailing winds blew low-hanging air masses of factory emissions (mostly SO2) towards China's east coast.
answer:
SO2


question:
What is reduced by playback equipment?, Context: Due to recording mastering and manufacturing limitations, both high and low frequencies were removed from the first recorded signals by various formulae. With low frequencies, the stylus must swing a long way from side to side, requiring the groove to be wide, taking up more space and limiting the playing time of the record. At high frequencies, hiss, pops, and ticks are significant. These problems can be reduced by using equalization to an agreed standard. During recording the amplitude of low frequencies is reduced, thus reducing the groove width required, and the amplitude at high frequencies is increased. The playback equipment boosts bass and cuts treble so as to restore the tonal balance in the original signal; this also reduces the high frequency noise. Thus more music will fit on the record, and noise is reduced.
answer:
treble


question:
What contributed to Estonia having one of the highest per capital income levels in Eastern Europe?, Context: Estonia has had a market economy since the end of the 1990s and one of the highest per capita income levels in Eastern Europe. Proximity to the Scandinavian markets, its location between the East and West, competitive cost structure and a highly skilled labour force have been the major Estonian comparative advantages in the beginning of the 2000s (decade). As the largest city, Tallinn has emerged as a financial centre and the Tallinn Stock Exchange joined recently with the OMX system. The current government has pursued tight fiscal policies, resulting in balanced budgets and low public debt.
answer:

location between the East and West, competitive cost structure and a highly skilled labour force