occur|occurs|occurred|occurring

在东行车道发生了一起涉及超过十辆车的事故。

An accident involving over ten vehicles has ___occurred in the east-bound lane.

如果在服药期间出现任何这些症状，请立即咨询医生。

If any of these symptoms ___occur while you are taking the medicine, consult your doctor immediately.

当月球经过太阳和地球之间时，就会发生日食。

A solar eclipse ___occurs when the moon passes between the sun and the earth.

试管中正在发生化学反应，产生明亮的蓝色火焰。

A chemical reaction is ___occurring in the test tube, producing a bright blue flame.