Here's why:

* DNA to RNA: DNA uses the bases adenine (A), guanine (G), cytosine (C), and thymine (T). RNA uses the same bases except for uracil (U) instead of thymine.

* Base pairing: In both DNA and RNA, bases pair up according to specific rules:

* Adenine (A) pairs with thymine (T) in DNA, or uracil (U) in RNA.

* Guanine (G) pairs with cytosine (C).

* Complementary strand: The complementary strand has bases that match up with the original strand according to these pairing rules.

Therefore, if the original DNA strand is "T C G A," the complementary mRNA strand would be "A G C U."