Which of the following correctly uses the setInterval() method to call a function named count-down() every two seconds?

a.
```
var interval = window.setInterval("countdown()", 2);
```

b.
```
window.setInterval(2000) = countdown();
```

c.
```
var interval = window.setInterval("countdown()", 2000);
```

d.
```
window.setInterval("countdown()", 2000);
```

c.
```
var interval = window.setInterval("countdown()", 2000);
```