Historical Linguistics and Language Families 3, Exercise 3: Romance k

Two sets appear: k : k (in corpo, crudo, catena, cacciare and their Spanish
and Portuguese counterparts) and k : [ʃ] (the French words chaîne, chasser).

French [ʃ] appears only before a. The two sets therefore occur in different
contexts: that is complementary distribution. Because the difference is
explained by the following vowel, both sets can reflect a single
proto-phoneme, *k (Latin c), which changed under a condition in French.

Step shown: step 3 of the outline, "find complementary distribution".
(The source states that French [ʃ] appears only before a; this exercise does
not check that claim against the French words beyond the table.)