Combine data from two columns into one column

I have a scenario wherein I have to combine two columns into one . Its SQL Server 2005
Below is the example :
CREATE TABLE Table_Source
(
PK_ID INT IDENTITY(1, 1),
Col_A VARCHAR(2000),
Col_B TEXT
)
CREATE TABLE Table_Destination
(
PK_ID INT IDENTITY(1, 1),
Col_C VARCHAR(MAX)
)
Sample data for both the tables:
Table_Soruce :
PK_id Col_A Col_B
1 Abc Null
2 Pqr Null
3 Null XYZ
4 Null EFG

I want to have data in Table_Destination in the below manner :
Table_Destination
PK_id Col_C
1 Abc
2 Pqr
3 XYZ
4 EFG
Can anyone throw a script to achieve this scenario. Table_Source will have million of rows to fetch from.
Thanks for your help