Modern words starting with cua- were qua- until the Academy's 1815 orthography update.

It is also worth noting that the use of qu for /k/ before e and i results from the generalized loss (according to Penny (1991: 83-4), at a period prior to the development of written Spanish) of the glide [w] from reflexes of Latin QU-, as in the case of QUINDECIM, which became Spanish quince ('fifteen'). Before stressed /a/, however, the glide was not eliminated, and it was only in 1815 that the Academy replaced qua- by cua-. Thus modern cuando [kwándo] was written as quando until the last century.

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ETA: Also, Cuál es vs Cuáles is fucking hard to distinguish until you get used to accents and can understand the whole sentence more quickly.