Because film is______to light it must be kept in darkness to avoid exposure under the sun.

It is hard to explain what a painter is saying, because ______ .A．most painters do not exp

According to the author, the concept of "aggression" ______.A．is hard to define because it

Barack Obama is urging to extend a payroll tax cut because it______.A．is set to expire on

Natural gas is used in Bremo to heat water because______.A．it produces more steamB．it caus

It is difficult for the woman and her friends to change dates because______.A．it"s rather

So fast _____it is difficult for us to imagine its speed.A.does light travel thatB.light travels th