I really don’t know ____ .

_____ with so much trouble, we failed to complete the task on time

_____ to sunlight for too much time, as the doctors say, will do harm to one’s skin.

-- Not knowing what style of iPad to choose, _______.

“XiJinping's report, ” a smile on his face, he continued, “_____ to several hot topics, discusses how to increase peasants' income.”