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Bubbles of gas have great difficulty in getting out of the magma.
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Driving forces are difficult (or hard) to measure accurately.
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Such suspended droplets are hard to remove from the gas stream.
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Even experienced economists may be hard pressed to guess the impact of new sources of a chemical.
Русско-английский научно-технический словарь переводчика. Михаил Циммерман, Клавдия Веденеева. 2003.