Choose the alternative that fills in the blank....

Questão de Inglês da banca Aeronáutica aplicada no concurso EEAR (2014). Confira a resolução completa abaixo:

Read the excerpt and answer question 42.

"Vuvuzelas were ______ popular during the 2010

World Cup that they were selling as many as 50,000 of

them a month".

(Adapted from www.cnn.com)

Choose the alternative that fills in the blank.