Here's why:

* Transcription is the process of copying DNA into RNA. RNA is single-stranded, while DNA is double-stranded.

* Base pairing in transcription is between the DNA template strand and the newly synthesized RNA molecule. The RNA molecule is built using the DNA strand as a template, with the following base pairing rules:

* Adenine (A) in DNA pairs with Uracil (U) in RNA.

* Thymine (T) in DNA pairs with Adenine (A) in RNA.

* Guanine (G) in DNA pairs with Cytosine (C) in RNA.

* Cytosine (C) in DNA pairs with Guanine (G) in RNA.

In summary, the base pairing rules for transcription are:

* A (DNA) - U (RNA)

* T (DNA) - A (RNA)

* G (DNA) - C (RNA)

* C (DNA) - G (RNA)

Note: Remember that there is no thymine (T) in RNA. It is replaced with uracil (U).