In a stress test for elite athletes a physical system is used to measure the performance of the individual. Indicate which of these signals are analog signals:

a$.V{O}_2=>$ Type: Liters$/$minute; Range: $[2-7\mathrm{.}51/$min$].$

b$.$ Respiratory Rate $=>$ Type: Numerical; Range: integers from $8$ to $40.$

c$.$ Heart rate $=>$ Type: Numeric; Range: Integers from $35$ to $300$

d$.$ ECG $($Electrocardiogram$)=>$ Type: Voltage; Range: $[0\mathrm{.}5-4$ mV$].$

e$.$ Activity level $($accelerometer$)=>$ Type: Current, Range: $0-7mA$

f$.$ Weight $=>$ Type: Weight; Range: $[25-100$ kg$]$