Identify which signals among the following are analog signals used in a stress test for elite athletes to measure individual performance:

a$.$ VO$->$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: $[0-7$ mA$]$

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