Read the passage about bird food and bird watching. noted scientist dr. elsa birde described the effect of adding a mixture of dried apples and raisins to the previous feed of sunflower seeds, millet, thistle seed, and cracked corn in bird feeders in her yard. over three years, dr. birde observed that the new mixture increased the number of songbirds at her feeders, especially during the months of april and may. as she has done for several years, dr. birde recorded, at the same time each day, the number of birds at each of her four feeders. she noted that the third year using the mixture coincided with the largest increase. there was a twenty-five percent increase in the number of songbirds over times when this mixture was not used. dr. birde also noted that in the third year she used a specific ratio of the ingredients, carefully measuring, which she had not done in previous years. the third year also had a warmer than normal spring, and a construction project began in a neighborhood near dr. birde’s. dr. birde is now going to produce this formula for sale and advertise to customers that they may see the same twenty-five percent increase in songbirds at their feeders. dr. e. birde, journal of scientific bird feeding
which might be a cause for the increase in songbirds at the feeders?
a) the time of day that the birds were counted at the feeders
b) the addition of dried apples and raisins to the formula
c) the temperature changes each year
d) the increased number of bird feeders