Here's why:
* "Is washing" is in the present continuous tense, indicating an ongoing action.
* "Mowed" is in the simple past tense, indicating an action that happened in the past and is now finished.
The shift in verb tense from present continuous to past tense creates a jarring effect and makes the sentence grammatically incorrect.
Sentence A does not have a shift in verb tense. Both verbs, "entered" and "ran," are in the simple past tense.