D. By being the first event in a sequence. (‘Verb + le了 + unquantified/unmodified object’ means ‘after [doing] ...’. It must be followed by another action.)
4.1 Míngtiān wǒ chī-le wǎnfàn jiù qù xuéxiào.
Tomorrow I will go to school after I eat dinner.
If Sentence 4.1 ended at wǎnfàn晚饭, that is Míngtiān wǒ chī-le wǎnfàn明天我吃了晚饭, it would mean ‘Tomorrow after I eat dinner’, which is not a complete sentence.
4.2 Zuótiān wǒ chī-le wǎnfàn jiù qù xuéxiào le.
Yesterday I went to school after I ate dinner.
The sentence-end le了 in 4.2 is a modal particle signifying the completion of the second action. The only difference between 4.1 and 4.2 is the use of the modal particle le了 at the end of 4.2. In 4.1 neither action has taken place yet, while in 4.2 both actions have. However, the aspect particle le了 in both sentences signifies that the verb it is attached to is the first action in the sequence.