we're not beginning until $9$:$00.$

Which option corrects the run$-$on sentence?

a$.)$ We usually have class promptly at $8$:$00$; today it's dififerent. We're not beginning until $9$:$00.$

b$.)$ We usually have class promptly at $8$:$00,$ but today it's different, we're not beginning until $9$:$00.$

c$.)$ We usually have class promptly at $8$:$00,$ today it's different, we're not beginning until $9$:$00.$

d$.)$ We usually have class promptly at $8$:$00,$ today it's different; because we're not beginning until $9$:$00.$