have been tumbling
Present perfect progressiveconjugation oftumble.There are other translations for this conjugation.

tumble

Iran increased production slightly in 2014, and Iranian officials said they could double oil exports if sanctions were lifted. More supply of oil, whose prices have been tumbling lately, would drive prices down further, which would benefit commodity importers.
En 2014, la producción iraní de crudo repuntó ligeramente y las autoridades de este país estiman que sus exportaciones de petróleo podrían duplicarse si se eliminan definitivamente las sanciones.
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