Many students cannot go to school as usual. It is because of the COVID-19 pandemic

1)Many students cannot go to school as usual. It is because of the COVID-19 pandemic.
A. Many students could go to school if it were because of the COVID-19 pandemic
B. Were it not because of the COVID-19 pandemic, many students couldn't go to school usual
C. Without the COVID-19 pandemic, many students could go to school as usual
D. But for going to school as usual, the COVID-19 pandemic would stop. 
2)Arthur entered the store. Then, he realized that there was danger.
A. Not until Arthur entered the store, he realized that there was danger.
B. It was not until Arthur entered the store then, he realized that there was danger

. C. Only when he entered the store then he realized that there was danger.
D. Only after entering the store did Athur realize that there was danger