I want to combine two records of datafram "df" with ID of "A" and "B" which lacks some data (NA)into one row with ID "C" (goal). I know matrix [ , ] can do this kind of work. But in the dataframe no row number is not available.

Below is my data.

```
df

ID    Y1    Y2    Y3    Y4    Y5   Y6

A     7     4     NA    NA    NA    NA

B     NA    NA     5    5     4     4 

```

goal:

```
ID  Y1    Y2    Y3    Y4    Y5    Y6

C   7     4     5      5     4     4

```