Here's why:

* Adenine (A) always pairs with Thymine (T).

* Guanine (G) always pairs with Cytosine (C).

So, following the base pairing rules:

* A in AGTT pairs with T

* G in AGTT pairs with C

* T in AGTT pairs with A

* T in AGTT pairs with A

This gives us the complementary sequence TCAA.