A series is given, with one word missing. Choose the correct alternative from the given ones that will complete the series.
Retina, National, Algebra, Radiator, ?

The series follows a pattern based on the last two letters of each word forming the beginning of the subsequent word:
* Retina ends with 'na' $\to$ National starts with 'na'
* National ends with 'al' $\to$ Algebra starts with 'al'
* Algebra ends with 'ra' $\to$ Radiator starts with 'ra'
* Radiator ends with 'or' $\to$ The next word must start with 'or'.

Among the given options, Ordinary is the only word that starts with 'Or'.