Convert two dimensional array into one dimensional array

Question

 

Decimal[,] num=new decimal[100,100];

 This is element of 2darray

1   2     3

4   5     6

7   8     9

10 11  12 

now i am want to convert it in one dimensional array

 1

4

7

10

2

5

8

11

3

6

9

12

 like this how to do?

please help me