Write the next sentence in this paragraph:

How to get free phone calls with smartphone apps
Notify your friends in advance that you're about to use an app to communicate with each other.
Tell them to download the app, which will be explained later in this article.
If your phone has a data limit, check if you still have at least 500 mb of internet data.

Choose from:
[a]. Knowing how many dollars you have is important. If you are willing to spend, it's worth it.;
[b]. Then, add it up and wait for someone to call. If you've 99 mb or more on your phone (if you would like to have a continuous internet internet connection), uncheck the limit.;
[c]. Add a cellular number to your data usage if you have one. If your device reaches a minimum volume, your data usage will go up.;
[d]. You might want to use a free public wifi if you have no data (or have little data left). On your phone, launch the respective app store.;
[d].