Aircraft structural units, such as spars, engine supports, etc., which have been built up from sheet metal, are normally

repairable

Explanation:

According to my research on all the different aircraft parts, I can say that based on the information provided within the question these parts are normally repairable. Although they will most likely only be repairable if the right technique is used, if the right technique is not used then the part will not be the same as when first manufactured.

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answer(d) sequencing and iteration;