Utilize the input text as a context reference, choose the right relationship between Airbus and aircraft from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

The slump in sales of very large aircraft has raised questions about how long Boeing and European rival Airbus Group NV can keep building double-decker planes, with 2015 a make-or-break year for both.

Answer: product_or_material_produced