The infusion pressure is low, the vein spasm and the inclined plane of the needle sticking to the blood vessel wall cause the infusion not to drip. Because the needle tip is still in the blood vessel, the needle itself is unobstructed, there is no resistance to squeezing the infusion tube, and there is blood returning when you let go; "There is resistance in squeezing the infusion tube, and there is no blood return when letting go" means that the needle has been blocked; When the needle slips out of the blood vessel, there is generally no anti-extrusion, and there is obvious swelling locally. Therefore, the statement of option A is correct.