[R] Combining columns

[Olu Ola]

 Hello,I have the following data
| colA | colB | colC | colD |
| NA | pumpkin | NA | Pumpkin |
| Cassava | NA | NA | Cassava |
| yam | NA | NA | yam |
| NA | Cherry | NA | Cherry |
| NA | NA | Pepper | Pepper |
| NA | NA | Mango | Mango |
| maize | NA | NA | maize |


All I want to do is to combine the first three columns in order to obtain the fourth column.
A way forward will be greatly appreciated.

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