how did it --- that you made so many mistakes in your homewo

正确答案：B come about
come about 发生
The flood came about as a result of the heavy spring rains.
春季的大雨造成了洪水泛滥.
occur to
…发生在
I hope no accident has occurred to him.
我希望他没出什么事.
come across
过来; 过访
Why don't you come across to our house this evening?
今晚你何不到我们家来坐坐?
被理解; 接受
He spoke for a long time but his meaning did not come across.
他讲了很久,但没把意思讲明白.
偶然发现,偶然遇见
I came across him in the street yesterday.
昨天我在街上无意中碰到了他.