YouTube was founded in $2005.$ By the summer of $2006,$ it was one of the fastest$-$growing sites on the web and had already become one of the top ten most popular websites. In $2006$ Google $($later Alphabet$)$ acquired YouTube for $$1\mathrm{.}65$ billion. Which of the following is the most likely corporate strategy behind this acquisition?

$($A$)$ A defensive measure to stop a competitor from acquiring the firm

$($B$)$ Acquisition of a complementary asset

$($C$)$ Unrelated diversification