D. By being the first event in a sequence. (‘Verb + le + unquantified/unmodified object’ means ‘after [doing] ...’. It must be followed by another action.)

 

4.1     Míngtiān chī-le wǎnfàn jiù xuéxiào.  

明天我吃了晚饭就去学校。

Tomorrow I will go to school after I eat dinner.

If Sentence 4.1 ended at wǎnfàn晚饭, that is Míngtiān 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 chī-le wǎnfàn jiù xuéxiào le.  

昨天我吃了晚饭就去学校了。

Yesterday I went to school after I ate dinner.

The sentence-end le 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 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 in both sentences signifies that the verb it is attached to is the first action in the sequence.