Develop a function called count signals that accepts two parameters, frequencies, which is a list of frequencies of the signals sent through the filters, and filter ranges, which is a list of ranges (each range is a list with exactly 2 elements) that have the lower and upper frequency bounds for each filter. This function should return the number of signals that pass through all filters. For example, if the function is called as count signals([8, 15, 14, 16, 21], [[10, 17], [13, 15], [13, 17]]), it should return 2.