There are quite a few English words that contain letters that are not suppose to be said including: debt and write. These left out letters can often tell us about the history and origin of the word and some places in the world has these which can tell us about who lived there at the time. This is called etymology.
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A participle has the following form: verb + -ing, and the only word in this sentence that takes that form is A, 'flying', which is the correct answer.
Although coal, hydrocarbons, iron ore, platinum, copper, chromium, nickel, gold and other minerals have been found, they have not been in large enough quantities to exploit. The 1991 Protocol on Environmental Protection to the Antarctic Treaty also restricts a struggle for resources.
