unidentifed offset errors php

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I made a simple quiz scoring using PHP but its coming up with unidentifed offset errors, but it works fine.errors:\[code\]Notice: Undefined offset: 0 in C:\Users\scorequiz1.php on line 17Notice: Undefined offset: 1 in C:\Users\scorequiz1.php on line 18Notice: Undefined offset: 2 in C:\Users\scorequiz1.php on line 19Notice: Undefined offset: 3 in C:\Users\scorequiz1.php on line 20Notice: Undefined offset: 4 in C:\Users\scorequiz1.php on line 21Notice: Undefined offset: 0 in C:\Users\scorequiz1.php on line 52Question 1. Correct.Notice: Undefined offset: 1 in C:\Users\scorequiz1.php on line 52Question 2. Incorrect.Notice: Undefined offset: 2 in C:\Users\scorequiz1.php on line 52Question 3. Correct.Notice: Undefined offset: 3 in C:\Users\scorequiz1.php on line 52Question 4. Correct.Notice: Undefined offset: 4 in C:\Users\scorequiz1.php on line 52Question 5. Incorrect.\[/code\]line 17 to 21:\[code\]$data[0] = $_POST["a"];$data[1] = $_POST["b"];$data[2] = $_POST["c"];$data[3] = $_POST["d"];$data[4] = $_POST["e"];\[/code\]line 50-56:\[code\]for($i = 0; $i < 5; $i++){ if($answer[$i] == 1) echo "Question " . ($i+1) . ". " . "Correct.<br/>"; else echo "Question " . ($i+1) . ". " . "Incorrect.<br/>";}\[/code\]my quiz is just like this. The options for question has the name "a" to "e" and the value to check for the right answer\[code\]<!-- question 1----><INPUT TYPE="radio" NAME="a" VALUE="http://stackoverflow.com/questions/10543561/1">10 days<BR><INPUT TYPE="radio" NAME="a" VALUE="http://stackoverflow.com/questions/10543561/1">40 days<BR><!-- question 2----><INPUT TYPE="radio" NAME="b" VALUE="http://stackoverflow.com/questions/10543561/1">True<BR><INPUT TYPE="radio" NAME="b" VALUE="http://stackoverflow.com/questions/10543561/1">False<BR>\[/code\]I'm not sure whats wrong