As Children Move Beyond the Telegraphic Period into More Sophisticated

As children move beyond the telegraphic period into more sophisticated use of grammar,morphemes such as -ing and -s are acquired before morphemes such as -ed and -'s.What does Brown attribute this order to?

A) -ing and -s are used more frequently by adults
B) -ing and -s are less semantically complex
C) -ing and -s are phonetically easier to pronounce
D) -ing and -s are both verb transformations, which are typically mastered prior to noun transformations