A, according to the voltage is proportional to the number of turns, it is known that when the indicator UA of voltmeter at both ends of secondary winding is 46V, the indicator U of voltmeter is less than =a66V, and A is wrong.

B. According to the expression, the effective value of power supply voltage is aa6V, and the secondary coil current q=4646= A is small, so the primary coil current is 6.aA, and the output power of AC power supply is P=aa6×6.a=44W, and B is wrong.

C. From A, we know that the resistance R is less than the partial pressure a6V, so R is less than =a66.a= less than 66Ω, and C is correct.

D. If the slider P of the sliding rheostat moves upward, the load resistance decreases and the current increases, then the input power of the ideal transformer increases, and D is wrong.

So choose: C.