Velocity of liquid hydrocarbon fuels in a pipeline can not be measured by magnetic flowmeters, because their __________ is very low/small.

Answer: Option B

Magnetic flowmeters work on the principle of electromagnetic induction.
This means they rely on the liquid being measured to have some electrical conductivity.

Here's a simplified explanation:
Imagine the liquid flowing through a pipe surrounded by a magnetic field.
If the liquid is electrically conductive, its moving through this field generates a small voltage.
The flowmeter measures this voltage and relates it to the flow rate.

Liquid hydrocarbon fuels (like gasoline or kerosene) are typically poor conductors of electricity.
In other words, their electrical conductivity is very low.

Therefore, if you try to use a magnetic flowmeter on a liquid hydrocarbon fuel, the signal generated will be too weak or nonexistent, making an accurate measurement impossible.

Let's look at why the other options are less likely to be correct:

Option A: Thermal conductivity: How well a substance conducts heat, while important in some applications, doesn't directly impact the operation of a magnetic flowmeter.
Option C: Specific gravity: This is the ratio of the density of a substance to the density of water. While it relates to the fluid properties, it's not the primary factor affecting measurement in this type of flowmeter.
Option D: Electrical resistivity: This is the opposite of electrical conductivity (how much a substance resists electrical current). While related, the more direct factor influencing the flowmeter's operation is the conductivity itself - whether it's high enough to generate a measurable signal.