问题：

[单选] On all vessels equipped with refrigeration units of over 20 cubic foot capacity, （）.

A . a gas mask suitable for protection against each refrigerant used, or a self contained breathing apparatus must be provided
B . it is the sole responsibility of the chief engineer to ascertain that all members of the engineering department are familiar with the use of gas masks or breathing apparatus
C . spare charges shall be carried for at least 50% of each size and variety of gas masks and/or self contained breathing apparatus
D . all of the above