A radioactive material is placed near a detector. The detector shows a count rate of 
. When a piece of card is put between the material and the counter, the reading decreases to 
. When an aluminium sheet is put between the material and the counter, the reading remains at . When a sheet of lead is put between the material and the counter, the reading decreases to 
. What is being emitted by the radioactive material?
α, β and γ- radiation
α and β-radiation only
α and γ- radiation only
β and γ- radiation only
