Which line of code should be used to make the following code snippet work?

```
var longString = "Great day, isn't it?";
var shortString = _____???_______
document.write("It's going ton rain to" + shortString);
```

a.
```
shortString = longString.substr(20, 7);
```

b.
```
shortString = longString.substr(6, 3);
```

c.
```
shortString = substr(longString, 6, 3);
```

d.
```
shortString = longString.substr(7, 3);
```

b.
```
shortString = longString.substr(6, 3);
```